The definitive marijuana guide from Cannabis UK

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Growing Marijuana in the UK

A complete guide to everything you wanted to know about growing marijuana in the UK, but were too stoned to ask.

This marijuana growing guide has been sourced from the internet. We did not write it but have found it to be an invaluable piece of information for UK based gardeners and I am sure that the information would be useful to growers across the globe. Thank you Sensi Sid for such a great grow guide.

Location, heating & ventilation
Lighting Systems & Techniques
Germination & Vegetative Stage
Flowering stage, harvesting & curing

Cannabis is an annual herb - that is to say it germinates, flowers, sets seed and dies in one year. Male and female flowering parts appear on separate plants (although it is possible to grow hermaphrodites).

Cannabis has two main varieties, cannabis sativa and cannabis indica.

Sativa is characterised by a tall growth habit, slender fingered leaves, lowish flower to leaf ratio and rather long flowering period.

Indica is characterised by a bushier growth pattern with wider fingered leaves, a higher flower to leaf ratio and a shorter, more predictable flowering period.

When smoked, sativa gives a clear, up, cerebral high while indica produces a 'stonier' more physical hit. In the last few decades, enthusiasts have created sativa/indica hybrids which combine the best features of both varieties.

The weather in the UK does not entirely suit the requirements of what is, essentially, a tropical plant. This guide attempts to show how a carefully controlled environment coupled with a suitable choice of plant variety can produce herb of the highest quality. In the author's opinion, herb grown organically tastes and smells better than hydroponically grown; therefore a compost-based method is fully described but fans of hydroponic methods will find that the principles outlined still hold good.

Location, heating & ventilation

The growing season in the UK is rather short. Cannabis is induced to flower when the daylight has reduced to about 13 hours per day. This happens sometime during September, therefore the plant has only 6 to 10 weeks flowering time before the first frost kills the plant.

September and October can also be cold and wet, increasing the chances of fungus and disease ruining the crop. There are varieties bred especially for outdoor growing in Northern Europe, but these tend to give poorer results than the best indoor or greenhouse types.

A greenhouse provides an environment that is warmer and drier than outdoors. This is beneficial because the crop can be started earlier and flowered longer with less chance of an early frost nipping the flowers. Flowering will still start at the same time as it would have done outdoors. There are many varieties that will grow both indoors and in a greenhouse with good results.

If you are buying a greenhouse solely for the purpose of growing cannabis, a 6ft by 8ft aluminium framed house will cost about £200; the same sum will buy you only 40g of good bud on the black market so it should be viewed as an investment rather than an expenditure. Avoid the galvanised steel framed type with stick-on flexible PVC covering if you want it to last more than two seasons.

Growing Indoors
Indoor growing allows the gardener to exercise complete control over the growing environment. Length of day, levels of light, temperature, humidity and nutrient are all adjustable. Arguably the best varieties are bred for indoor growing.

Setting up an indoor growing area Planning A space as small as the average wardrobe will provide enough bud for personal use (including a few friends). Heavy tokers may want to create a larger growing area - the following method can be scaled up to virtually any size.

Plan the grow room carefully before you start construction. Ask yourself a few questions:

Have I got access to several 13A sockets?
Can I get water to the grow room easily?
Will anyone be able to see light spillage?
Will anyone be able to hear the ventilation fans?
Will anyone be able to smell the plants (it's not called skunk for nothing!).
What can I do if I want to go on holiday?

Tell nobody about your horticulture unless absolutely necessary. Buy your supplies with cash and don't give your name or address to the retailer; in the past, some retailers have been busted and their mailing lists used by the police to raid growers.

Creating a space Find a suitable sized wardrobe or cupboard. If you want something bigger, construct a box with roughcut timber covered with plywood, chipboard or plasterboard. The box could extend from floor to ceiling, giving the impression that it is an integral part of the structure of the house.

The floor of the growroom should be some sort of waterproof tray - if you bring down the ceiling of the flat below, awkward questions will be asked! The floor (wardrobes and cupboards) should be strong enough to support the weight of your plants, pots and water-soaked compost.

Make a door big enough to allow easy access to the plants and equipment inside. You could try disguising the door of a floor to ceiling box such that it is not obvious that the growing room even exists!

Line or paint the room with something reflective; matt white seems to work better than a mirror finish. Remember that the inside will have to be cleaned periodically to remove dust, compost, mould and algae.

Ventilation is very important; plants need CO2 for photosynthesis and the temperature and humidity must be kept to an acceptable level. Louvres at the top and bottom of the space will sometimes be enough but it is better to arrange for some forced ventilation. Electronics shops like Maplin or Tandy sell 240v fans for around £15. Fans need to be kept running 24 hrs/day irrespective of the length of lighting period. If you can, take the fan exhaust through an outside wall to reduce the smell. Some growers report that an ioniser operating near the exhaust end of the air vent reduces the smell considerably. A circulating fan inside the room is desirable but not essential.

For healthy growth, the temperature needs to be maintained at 15C to 30C. Cannabis will tolerate temperatures as low as 5C and as high as 40C but these do not represent ideal growing conditions. If your grow room is in an otherwise unheated area (like a cellar or garage) you will need to install some sort of electric heating controlled by a central heating thermostat.

Lighting Systems & Techniques

The type of lighting used depends on the size of room and the type of growing method. Indoor growing requires the light intensity, ideally, to be between 2000 and 3000 lumens/sq. ft.

Tungsten filament bulbs are very inefficient and produce more heat than light; some sort of gas discharge lamp is required. There is much debate about the relative merits of the various types: fluorescent tubes, mercury vapour, metal halide and high pressure sodium. Mercury vapour and metal halide are useful during the vegetative growth stage; HPS are perfect for flowering and acceptable for vegetative growth; fluorescent tubes are general purpose. In the writers opinion, only fluorescent tubes and HPS need be considered as other types involve a change of lamp when the flowering stage is induced.

Fluorescent tube systems
Fluorescent tubes and their fittings are available in a range of lengths. Longer tubes obviously emit more light than short ones. Standard 1in diameter halophosphate tubes are rated at about 30 watts per metre and output about 2,500 lumens per metre (and 85 lumens per watt). Tubes are available in a range of colour temperatures: 6000K (daylight), 4000K (cool white), 3500K (white) and 3000K (warm white). Daylight tubes have more blue in their spectrum and warm white more red. Results are not much different whatever tubes you use but, if you have the choice, either use a mix of warm white and cool white or change from cool to warm white when the flowering stage starts. A quick calculation reveals that to achieve 2000 lumens per sq. ft you have to virtually cover the entire area of the room's ceiling with fittings. The fittings can get quite hot, so allow a space between them for air to circulate.

The light intensity drops quite dramatically as you move away from the tube; in fact, intensity falls with the square of the distance such that the intensity at two feet distance is one quarter the intensity at one foot. This means that plant growth is only really luxuriant up to a few feet from the tube. Fix the tube fittings to a board which is, itself, attached by cords or chains to the ceiling; this way the tubes can be kept as close as possible (a few inches) to the top of the plants. The board will have to be moved daily when the plants are growing well.

Fluorescents are most useful in situations where you are not growing the plants very tall. For instance, as a light source for cuttings (clones) or for the "sea of green" method.

HPS systems
HPS lamps are available in 150W, 250W and 400W sizes. The lumen output is, respectively, 15,000, 28,000 and 50,000. This amounts to between 100 and 125 lumens per watt, according to the size of lamp. Their colour temperature is quite low at between 2000K and 3000K; they emit a pinkish, golden white light which is similar to autumn daylight. Note that recently, a new type of HPS lamp has been introduced, the SON-T Agro, which has an extra 8% blue light added to the spectrum making it more acceptable for vegetative growth than the standard SON-T.

There are several advantages to HPS lamps:

The efficiency, in terms of lumens per watt, is high.
There is usable light at a greater distance from the bulb, compared with fluorescent tubes.
In most domestic installations, there are only one or two lamps to change as opposed to maybe twelve or more fluorescent tubes.
Lamps have a long lifetime of 20,000 hours or more (although the output drops, gradually, during the life of the lamp - it's best to change them after one year's use).

The disadvantages are:

The cost of an entire system comprising control gear, lamp and reflector can be several hundred pounds.
The colour temperature is not ideal for vegetative growth although most growers report acceptable performance.
HPS lamps should be kept at least 18" from the plants to avoid leaf scorch or even fire! Water should be kept well away from the lamps as they can explode if splashed.

The lighting system needs to be switched automatically so that the plants receive 18 hours of light per day during vegetative growth and 12 hours per day when flowering. Stores such as Currys and Argos sell timers, which plug in to an ordinary 13A socket, for less than £20. They claim to be able to switch 13A but inductive loads such as HPS lamp ballasts can burn the contacts quite quickly. Use the timer to operate a mains relay with a higher current rating, e.g. 30A.

For larger or more permanent installations use a surface mounted timer such as those used in central heating systems. These may have more than one timed outlet which may be useful if you are operating a two shelf "sea of green" setup.

Keep electrical circuits well away from water. Make sure that electrical wiring is installed following all the conventions used by the professionals, i.e. use cable of the correct current rating, use the correct rating fuses and earth all metal components (like control gear boxes) that could be touched.

Keep anything flammable away from hot lamps. If you can arrange it, fix a switch near the front door which acts as a master control switch for the whole grow room's systems. If you have a visit from the landlord or the boys in blue then you can shut down the lights and fans easily before anyone hears or sees anything suspicious.

Germinating seed

All plants start, originally, from seed. Choose a variety that is advertised to grow in the conditions that you have planned. Good seeds will be firm to the touch and not 'pop' when squeezed between thumb and forefinger. Avoid those which look undersize or very pale in colour.

Some books advise soaking the seeds between sheets of blotting paper and then transferring to a growing medium when they have sprouted; this is not strictly necessary, just poke the seeds, individually, into small plant pots containing a commercially produced seed compost to a depth of about 1cm. Water the compost, until it is damp - not sodden, with a solution of traditional copper fungicide or Cheshunt Compound. Place in a warm (15C to 25C), light place out of direct sunlight, keep moist and they should show their first true set of leaves within three weeks. At this stage, transfer the seedlings with as much of the seed compost as possible to individual containers, being careful to handle them by the leaves (not the stem) and avoid as much root damage as possible.

If you use small, biodegradable peat pots for sowing then the entire pot can be planted into the bigger container. Some growers transplant several times using progressively bigger pots but I find it better to transplant to the final growing container as soon as possible. Anyone not familiar with germination techniques should practice on bird seed or fish bait hemp before attempting to grow their very expensive 'Sensi' seeds!

Many people disagree with the method described above and say they have always found it easier to germinate seeds by placing them between 2 sheets of damp tissue paper in a tub of some sort in a warm position such as an airing cupboard or near a radiator. You should check it regularly and within a couple of days a small root should be poking out of the seed. Then you can place it in a potting composting in a small plant pot - root down  in a small hole with the seed part just below the surface of the compost.

The vegetative stage
Most varieties need to be grown for about 40 days minimum before they are mature enough to produce flowers. Of course, you can grow them for longer and produce much bigger plants but this is not always convenient. Smaller plants are more easily hidden or disguised. A four metre monster will look rather obvious between the rows of tomatoes!

During the vegetative stage, the plants need to get as much light as possible for as long as possible. This precludes starting your outdoor crop in September. Mid, to late May is usually a good time to plant out your seedlings.

Choose a growing site which faces south (for maximum sunlight) and is not overlooked by nosy neighbours. Plants can be disguised by growing between other tall plants such as runner beans or tomatoes.

Prepare the soil in your growing patch by digging in a bucket full of garden compost or well-rotted farm manure per planting position a few weeks before planting out. If the soil is sticky clay then dig in some horticultural grit to prevent the soil becoming waterlogged.

Seedlings that have been sprouted indoors require a period of 'hardening off' before planting outside. This involves gradually acclimatising them to outdoor conditions over a period of several weeks. Start by leaving them outside in a sheltered, sunny spot during the day and bringing them in again at night. Gradually increase the time they are outdoors until they are outside 24 hours a day. Without hardening off, the plants will suffer shock and their growth will be temporarily retarded.

Plant out with a space of at least a metre between each plant and water in well. There should be no need for extra fertiliser until the plant reaches at least a metre in height. In some areas, rabbits and deer can be a nuisance; the only answer is to build a fence to keep them out.

Some varieties of cannabis can grow to 4 metres in height; if this is a problem they may need to be pruned or trained. Pinch out the growing tip when the plant has produced 4 sets of true leaves; this will make the plant branch into two main stems. Pinching the growing tips again will further increase branching. Be careful not to overdo this process as the plants are stressed by pruning and their development may slow down unacceptably or they may even exhibit hermaphrodism. Upper branches can be bent horizontal (carefully!) and tied to canes with garden twine. This encourages budding at the leaf nodes and reduces the overall height of the plant.

During the summer, the plants will need watering occasionally. Don't keep them waterlogged; allow the soil to dry out somewhat between waterings. If the soil has not been prepared with plenty of compost or manure, then apply a garden flower fertiliser such as Miracle Gro according to the directions on the packet. It is better to under-fertilise than to over-fertilise; if growth seems satisfactory, either use no fertiliser or quarter strength fertiliser.

Greenhouse growing is a significant improvement on outdoor methods. Plants will grow more rapidly under glass and the buds will suffer no wind or rain damage. The flowering period can be extended because of the few degrees of frost protection.

Unless your plant variety is known to be short and bushy, don't start too early or the plants will be pushing the roof off by September. Decent sized plants can be grown from a June sowing.

It is possible to use the soil in a greenhouse border but this may contain pests and diseases. A much better option is to use commercially produced potting compost in containers. Use a general purpose soilless compost like 'Levingtons' and add about 30% grit, perlite or vermiculite to improve aeration and drainage. Perlite and vermiculite are better for aeration but are light in weight, some added grit will make the pots heavier and less likely to fall over. 12" pots seem about right for most plants but you can use bigger ones if you have the space.

Large plants will transpire large amounts of water when the weather is warm and sunny, so be sure to water regularly. Be careful not to over water; let the top few inches of compost dry out before watering again. In some areas, the tap water has high levels of calcium carbonate dissolved in it (hard water) and this can lock up nutrients in the compost. Use rainwater collected from the greenhouse guttering where possible. If you spend weekends away, it may be wise to invest in an automatic watering system. Ventilate the greenhouse in hot weather to keep the temperature and humidity down; high humidity can promote the growth of fungal infections. Buy an automatic roof vent opener (£15 to £20) and, if you can afford it, a louvred pane and automatic opener (£50 to £60).

Soilless composts have enough nutrient added for several weeks growth but you will need to feed with Miracle Gro or similar when this is exhausted. Organic growers can add blood, fish and bone mixture to the compost or water with fish emulsion. Use caution with fertiliser and don't over feed. Dilute to half or quarter strength to be safe.

In open ground, the wind thickens the plant's stem by bending it back and forth. In the greenhouse the plants may grow less substantial stems so tie the main stem to a cane with garden twine to stop it falling over. You can also train the main stem(s) by tying them to a horizontal cane; this will induce budding along the stem's length.

Automatic mains timers should be used to switch the lamps on and off. Some indoor growers keep the lights on for 24 hours per day although 18 hours should be sufficient and may produce sturdier plants (and result in a smaller electricity bill!). Make sure that the dark period is truly dark; just a few minutes of light interrupting the plants' "night" may induce hermaphrodism.

Plant your seedlings in suitable containers such as 10" or 12" diameter flower pots. Square section pots will waste less floor space than round ones. If you use old food or detergent containers then make sure that they have drainage holes in the base and that they are opaque. Follow the advice for compost, watering and feeding from the greenhouse section.

Main stems will tend to grow spindly under artificial light; tie them loosely to canes to stop them falling over. The lower stems will grow weakly if they are too far from the light source; pinch them out if they look too spindly, they may be used for cuttings. Grow the plants on until they are about two-thirds the intended final height before inducing flowering.

Growing media and feeding
In the opinion of the author (and of the Dutch seed suppliers, Positronics) cannabis grown organically has a superior taste and texture than that grown hydroponically (i.e. without soil). Organically grown plants seem also to be less susceptible to pests and diseases.

The choice of soil based (e.g. John Innes ) or soilless (e.g. Levingtons) compost is largely a matter of taste. Soilless composts are probably easier to carry, mix and handle than soil based.

Whatever your choice of compost, at this stage the pH should be neutral (around 7); this can be tested either by using a pH probe available from any good garden centre or by using litmus paper.

The texture of soilless composts will probably need to be altered because, straight out of the bag, they tend to get too waterlogged. It is important for the roots of the plant to get oxygen and for this reason about 30% to 40% horticultural grit, perlite or vermiculite should be added to open up the texture of the compost. Never use builders sand or anything you brought back from a seaside holiday as it will be either too salty or too alkaline. Perlite is a light, puffed volcanic glass and it is potentially dangerous to breathe the dust particles given off while handling it; therefore you must thoroughly wet it before mixing it. Vermiculite is an expanded mica in granular form; it presents the same risk as perlite so take the same precautions.

Plants need to feed as well as breathe. Growing plants need an adequate supply of nitrogen, phosphates and potash together with small amounts of the trace elements Mn, Fe, Mg, Cu, Zn, Mo, S and B. Nitrogen is necessary for healthy leaf growth, phosphates for healthy roots and stems and potash for producing flowers.

Commercial potting composts have enough nutrients added by the manufacturer for several weeks growth. This can be supplemented by adding pelleted chicken manure, composted cow manure or blood, fish and bonemeal when the compost is first mixed. Follow the instructions on the packet and, if in doubt, use less rather than more. If you are not fussy about following strict organic method, a soluble, chemical plant food can be used instead; this is added to the water in the proportions stated on the packaging.

The percentage of nitrogen, phosphate and potash is stated on the packet. A 15-30-15 mix means that there is 15% nitrogen, 30% phosphate and 15% potash with the rest being an inert filler.

At the vegetative stage, a fertilizer high in nitrogen is required. When the plants start flowering, change to a formula higher in phosphate and potash. Trace elements are included in most commercial formulas or can be added separately by using a chemical mix or a seaweed extract. At the risk of boring the reader I say again, more damage is caused by overfeeding than underfeeding. 

The flowering stage

When the day length decreases to about 12 or 13 hours per day, flowering will start. For outdoor and greenhouse plants, this usually starts some time in September but can be earlier in some modern hybrid strains. For indoor plants this is whenever the grower adjusts the lamp timers. Indoor growers using fluorescent tubes may want to swap their cool white tubes for warm white at this point.

Flowering is induced when the level of a particular hormone reaches a critical point. This hormone is produced during the dark period (night) and destroyed by light during the day. It is important, therefore, to ensure that the dark period is totally uninterrupted by any light , even a few minutes could disturb this sensitive process.

After a week or two of short days, look out for the immature flowers where the leaves join the stems. Males look like little green, upside down clubs (the playing card type) about one or two millimeters across and females look like green, slightly hairy hemp seeds with one or two thick, white hairs protruding from the top.

Pull up and throw away the males (or use them for making hash oil) before the flowers open and pollinate the females. The idea is to produce female flowering tops with no seeds and false seed pods oozing THC-containing resins; this is called sinsemillia which means "no seeds". If you are using fertiliser, change to one that is low in nitrogen and high in phosphorous.

The flowering tops will continue to grow, in the hope of receiving pollen, for another 4 to 8 weeks and produce more resins as they get older. When the plants are mature then they are harvested.

Harvesting and curing
Growers often disagree about the timing of the harvest and most have come to the conclusion that it is largely a matter of taste. When the plants are actively flowering, the resins contain a higher proportion of THC than during the vegetative stage. During the later stages, when flower production has slowed, THC is degrading to other related chemicals such as CBD. It is thought that THC is mainly responsible for the "high" and CBD for the more physical, "stony" sensations such as lethargy. If you want a clear high then harvest when approximately 35% of the pistils (little white hairs) have turned red or brown. If, on the other hand, you prefer the heavier hit you get from something like pakki-black resin then wait until 65% of the pistils have changed colour.

Cut off the flowering stalks with scissors and hang them upside down in a cool, dry, dark place until they are dry enough for the central stem to snap when bent; this may take one or two weeks. Pack the dried buds loosely into zip top bags or airtight jars and store in a cool dark place (the fridge is fine for this). Do not crush the buds any more than you have to as the trichomes holding the little beads of resin will fracture resulting in earlier degradation of the THC. Similarly, do not store the buds in a freezer as this also causes trichome fracture. Flowering tops stored like this can remain fresh for many months.

In an emergency, fresh buds can be dried more quickly by hanging them over a radiator or by spreading them out on a baking tray and putting them in an oven which has been pre-heated to gas mark 1 and then turned off. Naturally dried buds have a much better flavour and are less harsh to smoke than quick dried ones, so try to be patient if possible!

A batch of seeds is unlikely to produce all female plants; even if it does, each individual plant is likely to differ slightly. Cloning can produce many identical plants from one mother with ideal characteristics. Ideally, the process needs to be done under artificial light, indoors.

First, decide on which female to clone. If it has just finished flowering, switch the lights to a 18 hour on/6 hour off cycle; this will usually persuade the plant to revert to a vegetative growth state.

Prepare enough 3" pots filled with a moist 50/50 mixture of commercial seed and cutting compost and horticultural grit. Water them with a copper fungicide solution until the compost is damp, not sodden. Poke a pencil-diameter hole, about 4cm deep, in the middle of each compost filled pot.

Select healthy shoots about 10cm long, remove most of the bigger leaves, and then cut off the whole shoot with a scalpel or razor blade. Immediately drop the cuttings into a bowl of cool water so that the cut ends don't dry out. When all the cuttings have been taken, remove the bottom 1cm from the first cutting, with a scalpel, while it is still under the surface of the water. Remove it from the bowl and immediately dip the end into hormone rooting powder (or solution). Now, place it in the hole in the compost and lightly press the surrounding compost such that the stem of the cutting is held snugly in place, trying to avoid rubbing off the rooting hormone. Repeat this process with the other cuttings.

Put the pots in a propagator or a polythene covered, waterproof box. Put the propagator under artificial light set to a 18 on/6 off cycle. The idea is to maintain a fairly high humidity around the cuttings so that they do not lose too much water through their leaves while they are still growing roots. If the grow room is hot, it may be necessary to increase the ventilation in the propagator or box to avoid fungal growth. The cuttings will not need watering very often as most of the evaporating water is trapped in the propagator.

After a few weeks you should see some new growth at the tip of the cuttings. At this stage, gradually increase the ventilation in the propagator over a period of a few days so the cuttings acclimatise to the lower humidity of the grow room. When a healthy root system has developed, transplant the cuttings to their final growing positions for the vegetative stage.

Sea of Green
Sea of Green is a method developed by indoor growers to increase the frequency of harvests. The basic principle is to grow a larger number of small plants for a shorter length of time than the traditional method.

The growing area is arranged as a series of two or more shelves. Using a two shelf system as an example, the upper shelf is equipped with fluorescent tubes. This is used to grow a number of clones, vegetatively on a 18/6 daylight cycle, until they are big enough to flower. At this point they are transferred to the lower shelf which can be equipped with either HPS or fluorescent light. The lower shelf is operated on a 12/12 daylight cycle and forces the plants into flowering.

The shelves should be shielded from light spill from the adjacent shelf, either by light proof curtains or doors. Each shelf should have it's own timer, of course.

Clones of a single variety should be grown such that the growth rate is the same for each plant. By experiment, it is possible to determine the shortest length of time the plants have to be grown vegetatively before flowering is initiated. The yield from each plant will be comparatively low but the yield per month from the available growing space should be higher that with other methods.

Variations on this theme include a three shelf system where shelf one is used for rooting cuttings, shelf two for vegetative growth and shelf three for flowering.

Bugs and diseases
Keep an eye out for bugs which may attack your plants. If you use an insecticide, be sure to use one based on natural active ingredients, such as pyrethrins; after all, you will be smoking it sooner or later - don't poison yourself.

Aphids can be washed off with a soft-soap solution - cover the plant pot with a polythene bag, tied loosely around the stem, to avoid too much soap getting in to the compost. Whitefly can be controlled by spraying with permethrin at three day intervals until eradicated.

Spider mites can be a real problem when growing indoors. They like hot, dry conditions so try and keep the humidity level up to discourage them. Unfortunately, fungi and moulds like high humidity so, like most things in life, a compromise has to be reached.

Botrytis (Grey Mould) likes humid conditions. Cut off and destroy any infected parts; reduce the humidity by increasing the ventilation and, if necessary, spray with a fungicide which is safe for use on vegetables (read the packet).

Avoid using chemicals whenever possible, especially if you are nearing harvest time. Some insecticides and fungicides are highly toxic, in contrast to THC, and may cause you serious damage. Large scale growers use parasitic insects to control pests but this is probably impractical for the cupboard grower.


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