It’s difficult to grow cannabis outdoors and in greenhouses during the winter. Colder temperatures, less light, and increased humidity all present challenges for cultivators. This past winter, one initiative of Dark Heart’s breeding program was creating new winter strains that will perform for farmers. From a grower’s perspective, a great winter strain is one that performs well and produces a quality crop during the winter. Do the same strains that thrive during the summer also grow well during the winter? If not, how can outdoor and light dep farmers be successful during the winter, when many strains are prone to mold, botrytis, and poor yields?

These are all questions asked by Dr. Kay Watt, botanical breeder at Dark Heart. Watt recently studied hundreds of flowering plants as part of winter trials in Dark Heart’s R&D wing located south of San Francisco. Both photoperiod and autoflower strains were considered.

Breeding with Intention

Watt explains “It’s like a Ferrari vs a Jeep. Think of regular strains growing during the summer or indoors like high performance race cars on a race track–the cars are strains and the track is the ideal, perfect grow environment. Now let’s say you’re an outdoor or greenhouse cultivator during winter. In this case, the grow is a lot less like a closed race track and more like an off road course–it’s more rugged. Factor in winter weather conditions, and you’re going to want an SUV instead of a race car. You want more rugged strains.” 

Watt is using the trial results to selectively breed with for seed production and to create new strains. Here are some of the traits she watches for to create strains that will thrive during the winter:

Fast Growth 

Fast growth under cold nights and temperate days is crucial. Short days and long nights means that there’s a reduced window for photosynthesis, or for plants to grow. Winter strains must be able to thrive in cooler, darker conditions and finish on time. When it comes to flower onset and finishing, autoflowering strains perform very well, because they finish by age instead of photoperiod.

Autoflowers finish quickly and improve each year due to breeding efforts.


Open Structure

“A key performance indicator is good internodal length to separate the buds and avoid stacking, as well as having a naturally open canopy structure.” An open flower and plant structure helps minimize mold and botrytis by allowing airflow throughout the plant. This is crucial during winter as rain, humidity, and lack of sun means that plants may be wet for extended periods of time.

Geographic Gene Pools

The source of the genetics is another factor that Watt takes into consideration. “Afghani genetics and surrounding areas have evolved in an area that’s mountainous with hot days and cool nights. These typically have short flowering times and cold resistance.” Meanwhile “Tropicals,” or genetics from near the equator, “have good rot resistance since they have evolved in humid conditions.” 

Color Helps Sell

It’s no secret that consumers enjoy colorful cannabis, and strains with color can help farmers succeed. Finished flowers that are purple, blue, or black improve crop quality through greater bag appeal. “Anthocyanins are a value boosting bonus,” Watt explains. “These are the same molecules that create fiery autumn colors in Maple trees, they’re also responsible for purple coloration found in cannabis.” In the pursuit of purple, several strains were propagated out of tissue culture storage, including Purple Cadillac (pictured) and Purple Kush.

Next Steps

Watt is using the data provided by the trial results to guide strain pairings. The best winter performers of each seed generation will be selected and bred, and the best performers will be tested within Dark Heart’s cultivation network.

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