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best strains to grow in high heat

Mandala Heat Resistance & Tips for a Hot Grow

We breed strains that are heat resistant and used to thriving under higher temperatures. This beneficial trait has been thoroughly verified by growers in the past years who grow indoor in the summer or outdoor in hot areas. In particular, a good measure of heat resistance is important when you cultivate in a homebox or closet.

There are several factors that make Mandala strains perform better.

Genetics

Breeding

Adaptation

Hybrid vigour

Genetics

We use landrace (heirloom) cannabis genetics from several regions that are located in southern latitudes despite lying „north” of the continent itself. In countries like India and Nepal high temperatures, arid conditions, and intense UV light are prevalent in high-altitude mountainous regions. In several South East Asian, African and S. American countries the warm-humid months alternate with the annual blistering hot-dry season. Cannabis growing in these places naturally acclimatizes to these conditions in order to survive and produce enough seeds for successive generations.

Breeding

Our breeding goal is to preserve the vital robust characteristics of the landrace cannabis genetics in a hybrid cross without compromising the other important features for modern cultivation. None of our strains, or phenotypes within them, exhibit the undesirable features carried by several landrace genetics. This is accomplished through years of careful selection and breeding to achieve a perfect balance between the power of a wild species and refinement of a modern hybrid. Not only the % of the landrace genetics within a cross, but also the ideal match between the parent lines must be investigated through many trials and test grows.

Adaptation

The beneficial aspects of the genetics need to be retained through adapting the new hybrids to harsh grow conditions. This strategy is not only necessary in the breeding phase to „weed out” weak plants in promising parent lines. It is also important to biologically condition successive generations so that they carry on the environmental information within the genes. We therefore breed under intense grow conditions that enforce the survival capabilities of the plants. Our methods of plant care have the objective of supporting systemic acquired resistance (SAR), which is a “whole-plant” resistance response that occurs following an earlier localized exposure to pathogens. Years of observation have led us to the conclusion that induced resistance is also possible towards extreme temperature fluctuations and other environmental factors.

Hybrid vigour

Heat and pest resistance is dependent on plant health and it’s „hardware”. The outstanding growth of Mandala strains ensures plant performance under stressful conditions such as high temperatures. Strong and profuse root growth, large and generous leaf canopies, thick and tough stems ensure an abundant and long-lasting supply of water and nutrients during periods of drought, heat, and low humidity. Mandala plants not only last longer in conditions of drought or severe heat before wilting, but also recover much quicker after watering/cooling down. The supply of nutrients is ensured for longer periods, reducing or eliminating symptoms of undernourishment, during hot spells. Whereas a heat susceptible plant will show slow or stunted growth, yellowing of leaves, wilting, leaf drop, hermaphroditic flowers, etc. our strains tend to grow and flower even at temperatures up to 50°C/120°F if necessary. This does not mean that temperatures above 35°C/95°F or other stressful conditions are desirable and should be sustained for cannabis cultivation. But, they may be beyond the control of the grower at least for a certain period of time (summer months, etc.), during which the plants must rely on their resistance for healthy development.

Germination & Plant Care During High Temperatures

It is imperative that your plants receive enough water when temperatures rise above normal levels. We are referring to the water supply in the root zone. It’s a waste of time to spray your plants with mineral water while the pots are too dry – and it also doesn’t help them obtain nutrients. Roots require a moist substrate in order to absorb the mineral ions and supply the plant tissue with vital nutrients. You also don’t want to drown the roots in water – just keep the substrate moist, but not soggy or waterlogged. Wait for the top layer to dry before watering. Ensure proper drainage in hydroponic systems.

Do not make the mistake of covering pots with plastic wrap or putting them in a closed propagator for germination because you are paranoid about the soil drying out. The high humidity, coupled with the lack of fresh air, can cause fungus to develop in the soil and the seeds rot before they can germinate. In many cases this may not happen – but the point is that the risk is high. Why should you want to multiply a risk instead of minimizing it?

Successful germination is ensured by placing your pots somewhere in a normal environment away from direct sunlight/excessive heat. Temperatures of 25-30°C are ideal. Propagators mainly serve a purpose for rooting cuttings. If you want to use one for germination you should follow basic guidelines to do it properly. This means that the lid should only partially cover the tray and it should be removed completely for an hour 2x daily. In this manner you can prevent excess humidity of 60% or higher and ensure a sufficient exchange of fresh air. The pots should never be soaked with water or stand in water. This also depletes oxygen in the soil and causes seeds to rot.

As long as the substrate is moist and does not dry out in the root zone Mandala seedlings/young plants can thrive in warm temps and low humidity. There is no need to worry if relative humidity drops to a very low percentage. We regularly grow seedlings under extremely low humidity (as low as 10%) with no negative effects. Leaf burn is caused by lamps hanging too low, soil drying out, and watering or spraying the seedlings/young plants with fertilizers, root boosters, or organic teas.

Regularly spraying young plants with mineral water can also cause damage to the delicate leaves from high levels of minerals in the water such as sodium. Check the contents of the water beforehand or use pure RO (reverse osmosis water). Tap water contains chlorine and other harsh chemicals and minerals that you don’t want to spray your plants with. We never spray our plants and instead ensure that they receive enough watering, decent potting soil, and an adequate sized container.

Common Mistakes

Many plant care mistakes occur because growers pick up bits and pieces of information here and there but do not really understand how, and if, they fit together in cannabis cultivation.

Cannabis is not an orchid and it’s not a cactus. It doesn’t like to be periodically drenched in water or grow in excessive humidity (60% +) like an orchid. It also doesn’t enjoy growing in a dried-up substrate or in a high mixture of perlite/sand like a cactus.

Below are some of the most frequent mistakes growers make and why these particularly affect the plants in a hot environment.

  • High amount of perlite/vermiculite/coarse matter (wood chips, etc.) in soil:

Ready-to-use quality potting soil is already perfectly mixed by the manufacturer for direct use. It has a balanced water-to-air ratio to prevent excessive drying out and to prevent soil getting too compact. Potting soil is based on a mix of various materials such as peat, compost, worm manure, with traces of perlite, coco coir, etc. Adding large amounts of perlite/vermiculite/coco coir (ie. 10% and more) creates a very dry substrate with many non nutritive pockets. This depletes plants of valuable minerals and considerably lowers the water retention capacity of the soil.

Rich soil = nutrients/water retention = fast root & plant growth = health = prolonged resistance

  • Bottom or top layer of Hydroton clay pellets or stones in containers:

Not a good idea and based on the false assumption that this is beneficial (for example for drainage/water retention). Hydroton clay pellets have a poor pH that is very alkaline (pH 8+). When you water through these from the top you „corrupt” the pH of the soil and cause minerals to become locked and unavailable for the plants. The result are symptoms of deficiency. Some manufacturers have now started offering clay pellets that are pH neutral. However, they still deplete the soil of potential nutrients, lower water retention capacity, and reduce available space for root development.

Roots grow down due to gravity. The majority of roots in a cannabis plant are to be found in the bottom half of the pot and in particular along the bottom (later, when the plants start to outgrow the pot the roots will fill it to the top). If the roots hit a dry layer of stones/clay pellets with many air pockets they shrivel, dry up, and the plant has less root mass to supply it’s top growth. Again, also valuable nutrients and water are lost, because the bottom is not layered with potting soil.

  • Overfeeding (too much fertilizer):

One of the biggest mistakes in a hot area is to fertilize excessively. High salt content in the substrate draws water out of the plant through osmosis. The plant suffers from overheating of the cells because it can’t transpire to cool down. The result is that plant cells die (necrosis). Leaves wither and drop, leaf and bud growth comes to a halt. Make sure you never feed more than 0.6-0.7 EC on soil indoor or 0.8 EC outdoor. Don’t feed more than every 8-10 days with any type of fertilizer. Be especially careful with potent organic fertilizers, like pure guano, that have quickly available nitrogen. Stop feeding the last 3 weeks of flowering and, above all, don’t feed at all if your plants are healthy and pumping out buds. Keep EC at 1.0-1.4 in hydroponics for growth and mature plants unless specific conditions (such as CO2 enhancement) require higher levels.

  • Underwatering:

Don’t let the bottom half of containers dry out completely. Water outdoor plants during the morning so that they have enough water stored when the midday heat strikes. Use water retention crystals in the soil mix for guerilla grows in arid places.

Water = nutrients/moisture = growth = health = prolonged resistance.

  • No trays underneath container:

It helps to water occasionally from the bottom as well. Water from top or bottom alternately and not at the same time to prevent over watering. When pots dry out quickly, or too much, the soil develops cracks – especially along the container edges. The water then quickly runs through to the bottom without being properly soaked up by the soil. This cause run-off and leaches nutrients out of the soil if it happens often. Trays catch the run-off and make it available for the plant. They also allow for even spread of moisture in the pots if the soil was very dry.

  • Insufficient container size:

Small containers dry out quickly and leave your plants stunted. In hot conditions give your plants more root space and substrate to prevent unnecessary drying out. If you are growing on a balcony or in pots outdoor make sure you have at least 2-6 gallon containers for your flowering plants. Read our SOIL GUIDE for more info on choosing a container size.

Bigger pots=more roots/water/nutrients = better provision = lusher growth = health.

  • Clogged drip system:

Above average temperatures quickly lead to clogged drip systems in hydroponics. The nutrient solution in drips evaporates leaving fertilizer salt residues behind that block free passage. Feeding tanks may also develop slimy residues if not cleaned and filled with fresh nutrient solution regularly. This will also clog the system. For some reason drips that are hardest to reach tend to clog the most often! So check your system regularly if all plants are receiving their nutrient solution – especially if you are using rockwool which dries up super-fast.

  • Weak ventilation:

Make sure your air intake and output fans have the capacity to keep temps as low as possible in your grow space. The performance indicated on the packaging is always less in real terms. Calculate 2-3 times more performance for your needs. Have a rotating fan on 24 hrs. to move air constantly and prevent stale, hot air pockets in the room.

  • Wrong lamp system/light schedule:

Switch to lamps with less heat emission (400W/cool tube instead of 600/1000W). Use a light rail or switch off one lamp during the peak heat wave. Organize your grow so that the veg cycle is before or after the hottest month. Start the light cycle in the evening when outside temp. cools down. Put your ballasts in a separate space – they also emit heat.

  • Poor pest control:

A strong pest attack weakens plant defenses and makes them more susceptible to the negative effects of heat. Several pests thrive under warm conditions and if left unchecked turn into a serious problem. In particular spider mites are a common pest during the summer and they withstand high temps and low humidity. Neem oil is a good biological pesticide and plant enhancer that should be sprayed as a precautionary measure and to control pest populations. Sticky traps keep flying pests under control such as adult thrips, white flies, black flies, and aphids. Don’t wait until it is too late. Think in terms of prevention and take action to keep your grow space clean.

Useful products

There are some products which can assist considerably during a hot grow. Here is our favourite, most reliable, value-for-money list*.
* * Mandala Seeds shall not be responsible for the use of any information herein presented and you must make your own decision as to its suitability and completeness for your use. We are not responsible for the use of a product mentioned and any effects or consequences it may have on your cultivation.

  • Worm manure

Increases the water retention capacity of potting soil and is especially important when growing in containers. Nourishes microorganisms in the soil, provides nutrients and many valuable trace elements (there are 100+ known trace elements beneficial to plants and these are not or hardly contained in peat based potting soil). Enhances aroma and flavour of your crop.

Mix 10-15% with your soil.

  • Compost

This is decomposed organic material that has been thoroughly composted. It has the same beneficial aspects of worm manure but can be mixed in higher amounts in your soil mix. Some compost can even be used pure.

  • Endomycorrhizal fungi

Increase plant health, nutrient availability and water retention. Mycorrhizal plants are more resistant to diseases, such as those caused by microbial soil-borne pathogens, and are also more resistant to the effects of drought. Water retention can be increased by up to 40%!

Recommended product: Mycor Flower Saver Plus

  • BioMagix (and the BioFilter)

BioMagix is a product in powder form pioneered and produced by General Hydroponics. One of the most meaningful new products in recent years. In hydroponics: this specific mix of bacteria decomposes the organic matter and debris in the nutritive solution and transforms this potential source of disease into a supplement of mineral salts readily available to the plants. For regular water containers: stale water and the spread of pathogens is prevented. Water can be held indefinitely in a tank until needed.

Substrates and soils watered with BioMagix treated water benefit from the micro-organisms that protect the root mass from pathogen fungi by surrounding the roots with a protective barrier. For best results one should use the BioFiltre in combination with the BioMagix. The BioFiltre also oxygenates the water/nutrient solution and „hosts” the micro-organisms.

  • Water retention crystals

An option for guerilla grows or if you are not able to tend your garden regularly. The products which can be mixed into the soil are most effective.

PS: A good grow book goes a long way in providing a firm basis of educated information. Some people spend hundreds of dollars on seeds and equipment and don’t consider picking up a cultivation book for 25$. but it helps to do that.

Expert advice on all common pests that affect marijuana crops indoor and outdoor – with quality photos!

Nutrients, water and pH are vital to cannabis cultivation. This easy to understand intro covers all topics.

Choose the right Mandala strain for your location using a highly detailed climate guide and special info’s.

Mandala Seeds breeds heat resistant strains. Several factors make our marijuana strains perform better: genetics, breeding, adaptation, hybrid vigour.

Examples of Heat-Resistant Cannabis Strains

Heat-Resistant Strains

Every strain is different. Some strains of marijuana are easily stressed by heat, while other strains would thrive in the exact same environment!

When it comes to strains that tend to be more resistant to heat stress…

  • Sativa, Haze, African and Hawaiian strains all tend to be more heat-resistant since they originate from hot climates.
  • In general, auto-flowering strains to be relatively sensitive to heat since they originated in Siberia, but some strains have been mixed with heat-resistant strains to make them more suitable for warmer climates.
  • Many Indica-leaning plants, which also come from cool climates, can be surprisingly sensitive to the heat.

Save your plants from the effects of heat stress!

Example of Heat-Resistant Cannabis Strains!

Kaya Gold

  • In addition to doing well in the heat, this strain is overall easy-to-grow and resistant to pests and mold
  • Has a surprisingly short flowering stage (8-9 weeks) for a Sativa-dominant strain, and doesn’t get too tall
  • Buds produce very heavy effects – more of a ‘brain-melter’ with couchlock than a good choice for social situations
  • Does well indoors, and grows exceptionally well outdoors (plus it is ready to harvest earlier than most other strains)
  • Great yields
  • Get Kaya Gold from Seedsman (ships to US)

The Kaya Gold strain thrives in the heat. This plant experienced many days over 100°F (38°C) yet plants were unfazed!

Sour Diesel by DNA Genetics

  • There’s a lot of different companies offering Sour Diesel, but this particular version is my favorite
  • This strain withstands heat and light levels that can destroy other plants
  • I’ve grown it a few times over the years and it always grows exactly the same no matter which grow light I use
  • Buds are consistently potent and high quality
  • Try out Sour Diesel by DNA Genetics

Sour Diesel is the plant on the left. It produced over 9 ounces on one plant.

This bud from that plant grew within 8″ (20cm) of a 600W grow light in the middle of a heatwave. The plant on the right suffered tons of damage despite being further away. On the other hand, besides a little heat foxtailing right on top the Sour Diesel buds came out huge and potent.

THC Bomb Auto

  • An auto-flowering plant that’s ready to harvest about 75 days from germination in my experience
  • I’ve grown it several times, and it always turns out well (though they all grow a little different)
  • This strain experienced temperatures up to 89°F (32°C) during the middle of summer, and never skipped a beat
  • Very potent strain, and good yields
  • Try out THC Bomb Auto

Examples of THC Bomb Auto at harvest

Auto Amnesia by MSNL

  • Another heat-resistant auto-flowering plant
  • Ready to harvest about 11 weed from germination
  • This strain has thrived in every condition I’ve grown it in, including some very hot days
  • Tends to be easy to grow and hardy
  • Beautiful buds, potency and overall bud quality
  • Try out Auto Amnesia

Here are some examples of Auto Amnesia plants I’ve grown.

Grown in a mini tent under a 100W LED grow light

Example of those buds after being dried and cured

Here are more Auto Amnesia plants. See the full grow journal.

Examples of Heat-Resistant Cannabis Strains Heat-Resistant Strains Every strain is different. Some strains of marijuana are easily stressed by heat, while other strains would thrive in the