Summertime, and the Growing Ain't Easy

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Indoor grows can be a nightmare in summer. Even if lights are timed to come on during the relative cool of the night – as many indoor growers will tell you is crucial – the hot months bring a whole host of problems. Most indoor grows rely on induction ventilation systems to regulate temperature, but when the air drawn in from outside is almost as hot as the air inside, this system fails dramatically.
Most Cannabis varieties can withstand a maximum ambient temperature of around 95ºF before systems begin to shut down, and anything above 85ºF will usually affect yield and quality of harvest if uncorrected. How well your plants will respond to heat depends on genetics, the stage of life they are at, and various other environmental factors. Cannabis is found in many hot areas of the world and the key to successful harvests is maintaining balance between the temperature and all other factors.
Humidity is one of these factors – a major one. Cannabis usually prefers a relative humidity (RH) of 60-70% (55% during flowering) to effectively maintain the transpiration cycle, but some strains can handle 80% or even higher. As temperature rises, plants drink and transpire more water in an effort to keep cool. Increased transpiration (where the stomata on the leaves open to release water droplets that often evaporate in high temperatures) leads to a dramatic increase in humidity. As air temperature increases, more water vapor can mix with it before being forced to condense or precipitate.
In an insufficiently ventilated room, the effect can become runaway. When saturation point is reached, the excess water vapor in the air simply condenses on any available surface, including the leaves that are attempting to lose it. When the humidity level in the air immediately surrounding the stomata is high, there is no steep temperature gradient to encourage transpiration, which slows and eventually stops. When transpiration stops, the plant’s internal temperature increases until it can reach a critical point, where growth stops and irreparable damage may occur. Ventilation and good air circulation are the best cures for this problem, but again – in a region where the outside air is also humid and hot, even the best rate of air exchange will do little to maintain correct conditions.

Some heat-adapted varieties can tolerate up to 120ºF if light intensity and irrigation are adequate, but this is most often seen outdoors, where air replacement is more than sufficient to ensure humidity does not get too high. In the average grow room, temperatures that high indicate that something is seriously wrong.
Cannabis that is flowered in consistently high temperatures often shows a loss of potency, with unusually light harvests of stretched, airy buds. Vegetative growth is often also stunted. Flowers and leaves close to lights will burn more easily and severely as air flow becomes less effective. In hot, dry conditions, spider mites and other insect pests can thrive.
Conversely, heat coupled with high humidity causes the plant to become susceptible to increased pathogen levels. Botrytis (otherwise knows as bud rot or gray mold) and other fungal infections can take hold. Whole, apparently healthy crops can be ruined almost overnight.
Solutions to increased temperature in grow rooms include: addition of CO2 to the atmosphere, A/C units, adding cool-tubes to lights or switching to cooler systems (such as LEDs), making sure your nutrient solution is constantly cool (more important for hydroponics growers), and buying up the entire stock of fans from every hardware store in the local area. The last is less a solution, however, than a desperate last resort by unprepared growers about to lose their entire crop….
With CO2 enrichment up to 1500ppm, it is believed that plants will actually grow faster in high heat, at least in the vegetative stage, and as long as light is ample and humidity is kept below around 60%. Increasing atmospheric CO2 in high temperatures allows the stomata to open and transpiration to resume, and according to some sources it also has the added benefit of killing many parasites, including spider mites. Using this technique has its risks, however – allowing the concentration to exceed 1800ppm can cause toxicity to the plant.
It is generally accepted that CO2 use to maintain rapid growth in high temperature should be stopped when flowering commences. In nature, flowering mostly occurs when the days begin to cool and shorten. By all accounts, your plants will not thank you for excessive temperatures at this stage, CO2 or not, and alternative methods to control conditions should be employed.
Temperatures can be lowered with the use of air conditioning, but this can cause costs to skyrocket as large systems consume much power, and can be expensive to buy and install. They are also noisy, which is a real disadvantage when attempting to maintain discretion. Water-cooled systems have the advantage of being quieter but will cause another hike in monthly bills, while possibly attracting the attention of more suspicious officials. Some A/C units also act as dehumidifiers, and these are especially worth consideration by those experiencing very hot and humid conditions. A small A/C and dehumidifier unit that is sufficient to cool up to three 1000w lights can cost as little as $300 new, but can use over 2000w in power. It is a cost that simply must be borne in some situations.
Cooled HPS lights can knock a few degrees off the mercury, but switching to LEDs can dramatically lower the temperature of your room. The lower power consumption of the light itself, when added to the savings made on cooling systems, presents an attractive solution. But initial cost of the top-end products remains so high as to be unaffordable to many in the short term. Growers are also still wary as LEDs have received poor press over the years, but recent advances have been rapid and results are becoming more and more impressive with time. As experimental LED grows continue, we will learn more about how they are most effective, but conclusions are already being drawn amongst enthusiasts that they may indeed be the solution to the summer heat problem.
In cold conditions they are in fact ostensibly pointless due to the need to use energy-consuming supplementary heaters. A set-up consisting of several lower-wattage LED lamps will provide superior light intensity and coverage to a single high-power light. Using this as an important guideline, growers are reporting tight buds and good yields in summer.
Paying attention to the genetic background of the strain you choose will serve you well – plants with a subtropical or tropical lineage will respond much better to increased heat and humidity than a temperate zone specimen. There is a variety for almost every climate zone, and finding the right strain for your own may be challenging but will certainly prove rewarding. Although this is obviously much relevant to outdoor growing, it will have some bearing on an indoor grow. Conditions inside still to some extent depend on external conditions, unless you do not rely on air drawn in from outside. Sativas found in hot, dry regions often have narrow leaves with high resin content which reduce transpiration. Long internodes, wide-spaced branches and loose flower structure allow greater airflow and less opportunity for damp spots in hot, humid conditions. Leaves are often somewhat wider to provide greater surface area for transpiration.
Cooler, humid conditions (such as are found in parts of mountainous northern India and Nepal) see indica varieties with shorter, stockier structures and very wide leaves. These are the strains that are most dependent on photo-period to flower: equatorial plants are used to a 12/12 light cycle so will continue to flower; ruderalis and some hemp varieties auto-flower based on age rather than change in day length. Cool northern conditions favor these hemp and ruderalis landraces, which are highly resistant to molds and fungi. When attempting to grow potent marijuana in these short summers, outdoor growers encounter many difficulties, and varieties bred to withstand cool and damp are often lacking in potency and yield. The best solutions are often compromises, and in this case there is great potential for some of the more potent ruderalis crosses to make a mark.
As long as your variety is correctly selected for your climate zone, you should have few problems outdoors. The more sun the better, as long as the plant is comfortable in other respects. As I mentioned earlier, Cannabis is designed to grow during the hot months and in much of North America your harvest is more likely to be affected by the fact that the season ends too soon for many strains to finish with full potency. The more south you look, the greater the variety that can safely grow outdoors. Some growers are even managing two harvests a year! Variation in day length throughout the year is much smaller in these areas – in California it’s around four-and-a-half hours, in southern Texas and Florida just three hours, compared with closer to eight in the northern states, and fourteen in Alaska. Yet there are strains that will grow in all these places, and just as a tropical sativa used to small day-length variation will be useless outdoors in Anchorage, a strain that really needs long days to prevent flowering will have a hard time growing in Key West.
Much of the continental US is capable of producing good harvests of indicas and indica-dominant hybrid strains, so sticking with these varieties is usually a safe bet. If you are in the south, you can choose sativa strains, but be careful with regards to humidity as there are massive variations between states. California is much less humid than Florida, it goes without saying, as although they experience similar annual precipitation, the westerly prevailing winds carry most of the moisture eastwards.
In the far north, strains such as Matanuska Tundra have been developed for the colder climate, though with many of these indica-dominant varieties, the tight bud structure means extra care must be taken to prevent mold and a lower RH is preferred. For cold, damp conditions, indica varieties selected for mold resistance (such as Dinafem’s Blue Hash strain) may be more suitable, and ruderalis hybrids should be considered. Frost in early spring is a risk for most of the US, so it is always wise to germinate indoors in February or March and transplant when conditions are favorable, often in April or May.
Beyond issues of climatic suitability, the risks associated with outdoor growing in summer are mostly to do with pests and diseases. Although a plant that flourishes in your climate zone may be well adapted to resist local parasites, this may not be total, and regular visits to ensure health will be necessary, along with a foliar spray every so often to guarantee peace of mind. Always ensure plants have adequate root space, as with strong sunlight outdoor growth can be rapid. In hot, dry areas, placing water-absorbing polymers in the soil reduces the need to water as frequently by releasing water steadily.
However you choose to go about your summer grow, the important concept to bear in mind is that all conditions must be right. Many strains will grow in less-than-ideal circumstances but it is a deeply unsatisfying feeling to spend months on a crop only to see substandard results. While it may not be possible to tweak every last variable to perfection, we still have a lot to work with when ensuring our crop remains trouble-free in the heat. If you’re serious about your horticulture, it should not be an impossible challenge to optimize your growing conditions. It may take some financial investment, but a lot rests on making the correct, informed decisions about the variety you will choose and the methods you will employ, and a solution should be within reach whatever your budget. Making sure that we check all the boxes and provide the best possible care to our plants is the least we can do – the rest is up to nature.

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