Overheating is one of the main causes of plant failure in greenhouses. A greenhouse must retain warm air during the cooler months, but it’s equally as important to release hot air during the warm months. When choosing a greenhouse, give consideration to ventilation. The glazing must have vents to ensure good growing conditions.
The combined area of all the vents when open should be at least a fifth of the floor area. There should be sufficient allowance for air to enter and exit the structure. Look for vents near the top of the structure and base vents for air intake. In addition to manually operated vents, it is worth paying for an automatic vent opener. This contains a cylinder of wax that expands and opens the vent when it’s warm, then closes it again when it’s cooler. This lets you be away from the greenhouse without having to worry about your plants sweltering during the hottest parts of the day. Some greenhouse designs also feature exhaust fans to prevent overheating during spring and summer months.
Once you’ve chosen your greenhouse, it’s time to pick the right spot to set it up in. Ideally, your plants will need 5-7 hours of direct sunlight per day. The more direct sunlight the plants get, the better the yields you’ll harvest. Morning dew will tend to get trapped in buds and may form mould over time, so direct sun from as early in the day as possible will minimise the moisture trapped in the buds and will reduce the risk of bud rot. Of course, this will only work if you have adequate ventilation in your greenhouse. So, you’ve got your greenhouse sorted, now what are you going to grow? If you’ve got your heart set on a regular photoperiod strain then you should know that the outdoor growing season lasts from April through to November, with the majority of strains ready to harvest from mid to late October. Seeds should be sown in early April, so that they are ready for moving outdoors from mid-May to early June. Start them indoors under a regular propagation light. As you get closer to the date that they will transplanted to your greenhouse, harden them off by taking them outside for a few hours a day; increase the amount of time they spend outside to get them acclimatised to the outside temperatures. The plants will vegetate through the months of May and June and will be triggered into the flowering photo period after the summer solstice – the longest day of the year – this is when the amount of daylight hours gets progressively shorter and the plants instinct is to put out flowers – buds – in order to reproduce. Most strains will take between 8 and 12 weeks to finish flowering, meaning that crops will be ready to harvest between the end of September to the start of November. Be warned that regular photoperiod strains can get HUGE when grown outdoors and it’s very easy to run out of head room in a small greenhouse, so you’ll need to spend time topping and training your plants. For growing in a domestic sized greenhouse in UK weather conditions, it’s a good idea to look at autoflowering strains. Autoflowers are great for outdoor growing in the UK. For those of you that don’t know, they are a non-photoperiod sensitive variety of Cannabis which – in layman’s terms – means that you don’t have to rely on the days getting shorter in order to get them to flower. They will automatically start to produce bud sites and begin flowering two to three weeks after germination, regardless of the amount of light hours they receive. This makes them a safe bet for producing some decent greenhouse weed, because you can basically plant them out when the weather is already nice and warm, safe in the knowledge that they’ll be ready to harvest within 60 to 90 days. Yes, it’s true that the plants will be smaller and produce a lesser yield than regular photoperiod sensitive varieties, but at least you’ll be guaranteed an outdoor harvest. The fact that autos are quicker to finish means that you’ll be able to pull off more outdoor harvests over the year, because autos begin flowering automatically, growing outdoors and starting a crop early in the year (weather permitting) means that you’ll be harvesting in the middle of summer. Overlapping your planting and harvest times by a few weeks or so means that, even in the UK, you could easily pull 3 outdoor harvests in a year! Or keep adding new plants to your greenhouse every few weeks and you’ll be harvesting on a regular basis come the end of the summer. The short growth cycle also allows you to start an outdoor crop later in the year. So, if we have a poor spring you can plant well into the summer months and still pull an outdoor harvest. If you’re situated in Ireland, the North of England or Scotland you can easily pull off a greenhouse crop. The Cannabis Ruderalis influence in the genetics of auto strains means that they are more resistant to the cold weather, disease and moulds (they grew up in Russia and Siberia after all!).
Just as with regular photoperiod strains, your plants will need to be started off indoors, but this doesn’t mean you’ll need to splash out on a full grow room set up. Get yourself a cheap high dome propagator, some light mix soil, some 10cm pots and a single T5 strip light. Sow your seeds in the pots filled with light mix, wait for them to germinate and then bring them on for a few days under the T5 lamp. Then you’ll need to harden them off for a few days; remove the propagator lid and keep the T5 just above the tops of your babies. Finally, it’s time to get them ready to go into the greenhouse. One of the big selling points with growing autos in your greenhouse, is that they will stay small in size. ‘Dwarf’ or ‘Low Ryder’ varieties of auto will reach no more than a foot and a half in height. Most of the auto strains (with the exclusion of the ‘Super Autos’) will remain small. This is perfect for hiding your plants in amongst pots filled with herbs or veg.
Even better would be to grow the autos along the centre of the greenhouse and place some vine plants, like tomatoes and cucumbers, up the sides of the greenhouse, this will break up the silhouette of the cannabis plants and keep them hidden from the neighbours. After you’ve chosen your seeds, the only decision left to make is how you’ll actually cultivate your greenhouse plants. If you decide to hand feed then planting out in large pots of soil or coco will cut down on the watering workload and also ensure that there is plenty of moisture available to the roots during warm days. As the plants are undercover, you are free to choose whichever nutrient schedule you wish, organic or mineral, safe in the knowledge that your expensive nutrients won’t get washed out of the pots by the rain. It’s tempting to run mains power into your greenhouse to power a hydroponic system but, before you do, take a look at the possibilities offered by some of the better passive hydroponic systems; they operate without pumps, timers or electricity, and they’ll equal the yields of traditional hydroponics.