You don’t always need to cold-stratify cannabis seeds, but there are some circumstances where knowing how will come in handy.
Decades of cannabis cultivation prohibition created a unique culture around growing our favorite plant. Although it thrives outdoors in a variety of global climates, many home growers and professional seed-makers learned and continue to grow cannabis in very controlled indoor environments.
In these managed settings, growers needed an ideal place for germinating cannabis seeds. Sometimes, even though a grower has set out the Welcome mat, seeds may still resist starting. This can be due to many factors, including genetics and how seeds were treated between harvest and germination.
The problem is that commercial, home, and wild-grown seeds can sometimes go into a state of self-preserving deep slumber, known as dormancy.
What is the difference between stratification and scarification?
Step 2, Day 1: Rinse seeds with a dilute solution of water and hydrogen peroxide
Step 6: Use either the “box of earth” or “fridge method” to chill seeds
Step 8, Day 6: Remove seeds from stratification temperature and go ahead with your grow plan
What is stratification?
With cannabis seed stratification, growers mimic the temperature fluctuations a seed might be exposed to throughout its natural seasonal cycle. Many of today’s popular cannabis varietals (‘strains’) were bred for growing yearly, living out their whole lives through the (sometimes quite short) summer. Their default ancestral “programming” is to mature in the cool of autumn, hunker down through the winter, and start up again in the warmth of spring.
What is dormancy?
Dormancy is one of nature’s diverse ways of ensuring at least some seeds from each generation survive to pass on their family genetics. Although it can be a hurdle for the expectant new parent of a fresh young cannabis plant, dormancy is not a major issue when you know some techniques for ‘waking up’ a sleeping seed.
Your goal is to reproduce some of the conditions that would signal to a seed that it has made it through the tough times. Now, it has the ‘go-ahead’ to emerge into its found environment.
Do cannabis seeds need cold stratification?
In general, no. Most high-quality cannabis seeds you’ll come across are products of a rapid cycle of controlled growth, harvest, and re-growth. They tend not to need cold stratification because, for many generations, their genetics haven’t faced the adversity of long survival in less-than-optimal or unpredictable conditions.
However, just like how dogs still sometimes show traits of their wild canine ancestors (howling at the moon, anyone?), domesticated cannabis seeds will occasionally act a bit ‘feral.’ Whether this is all the seeds in your batch or a single outlier, cold stratification is a way to recognize and accommodate their mistrust of a perpetual warm season.
What are the benefits of cold-stratifying cannabis seeds?
Cold stratification sends a very loud and clear message—treatment with cold signals that it is definitely not the right time to germinate. When conditions rapidly improve, seeds ‘understand’ they better get growing!
As the seeds are wet during this process, cold stratification also serves as a way of “hydropriming” seeds. This activates the germination enzymes necessary for the living cannabis embryo to finish maturing inside the seed shell. It makes seeds grow uniformly and speeds up their development as new sprouts.
Marijuana seedlings which are all the same height
What is the difference between stratification and scarification?
Stratification and scarification are two types of techniques for promoting seed germination.
To keep things simple, we’ll say:
Stratification is temperature treatment relating to the Timing of germination. By cold stratifying marijuana seeds, we are signaling the time to grow might be ‘now or never.’
Scarification is more about the Ease of germination. When we use scarification techniques, we are taking as much of the effort out of germination as possible, leaving more of the seed’s resources for the post-germination establishment of the young sprout.
Both address the protective characteristics a plant has evolved to ensure the best survival rate for an individual seed and amongst a population of seeds. Natural diversity exists even within a single generation from the same genetic donors (the “parent plants”).
When should you cold-stratify seeds?
If you have older seeds or weed seeds that took longer than others to sprout in the past (for example, seeds remaining from a pack you tried out in a previous grow cycle), you might consider cold stratifying.
Keeping extra seeds in a cool, dark, and dry space is good practice for long-term storage. People often use their refrigerators as a reliably stable spot to provide these conditions.
Food safety guidelines recommend that household fridges be set to a temperature range between 34-40°F (1-4°C) to slow or prevent the growth of potentially harmful bacteria and molds. While resting in this range does help plant seeds last longer, being stored in this environment can also encourage seeds to enter dormancy.
How to cold-stratify cannabis seeds
Researchers at the University of Oregon identified a reliable dormancy-breaking cold stratification method specific to cannabis. It involves hydrating (wetting) seeds and keeping them at 50°F (10°C) for five days before putting them into a welcoming germination environment.
Where can I cold-stratify cannabis seeds?
You can cold-stratify cannabis seeds in either a refrigerator or in a controlled setting that mimics the outdoors (see the “box of earth method” below).
The “cold” in cold stratification is a relative term. The recommended 50°F (10°C) is below what people usually consider a comfortable room temperature and much cooler than optimal germination and growth temperature, 70-85°F (20-30°C).
Home fermenters, craft beer enthusiasts, and wine lovers may already have a secondary fridge engineered to provide this ideal “cellar temperature.”
Barring that option (see what I did there?), depending on where you live, you may scout a spot in an unheated garage or basement. If unavailable, you can place a weather and animal-safe box partially filled with sand or potting mix outdoors to mimic springtime soil conditions. A balcony or unheated personal storage unit could be suitable if you live in an apartment.
Stratifying cannabis seeds outdoors
I’ve gathered some handy online tools to help you determine whether there is a good time of year to stratify cannabis seeds outdoors in your area. Indoor growers can use this information to schedule seed stratification before providing a quick transition to artificial “ideal summer” temperatures in a grow room or tent. For example, if you live in Alabama, you can cold stratify seeds outside in November, December, or January. In Maine, outdoor averages don’t rise to this range until May.
April temperatures in the contiguous United States and several of Canada’s bigger cities average in the cold stratification range.
If you live in the USA, you can use this online climate tool to see which months your state experiences temperatures close to these “ideal Spring” conditions.
In Canada, you can try this climate index site for local climate data.
The two best ways to cold-stratify cannabis seeds
There are two basic ways to cold-stratify cannabis seeds. Growers can use either method to germinate their seeds in paper towels or sow their cold-treated seeds directly into a growing substrate.
The “box of earth method” involves putting hydrated seeds into a container and half-burying this container in cool “soil” to simulate living through the natural transition between winter and spring. After five days, you will dig up the container and transfer the seeds to a hospitable, “summer-like” growing environment.
An example of a partially buried box containing cannabis seeds. You cannot and should not use scarification using this technique.
The “fridge method” is similar, but (if in a food, not beverage fridge) the temperature is lower and, as discussed above, maybe the temperature at which your seeds learn to stay dormant and resist germination. If you keep your wet seeds in this range, you may find greater success by combining stratification with a simple mechanical scarification technique. It is safer to do this kind of scarification on seeds that are kept in the colder fridge than in the “box of earth”/“cellar temperature” because competitor molds also find it harder to grow at the lower temperature.
An example of how you can place a box (normally containing cannabis seeds) within a fridge. You can use scarification using this technique.
Stratification: Step By Step
Time: 6 Days
Difficulty: Beginner to Intermediate (depending on how challenging it is for you to provide a sterile and consistently cool environment).
Step 1: Preparation
Decide on the most suitable place for your wet seeds to be kept in conditions close to 50°F (10°C) for five days, as discussed above. Give yourself an extra day before starting from Step 2 if you will use the “box of earth method.”
Collect the following:
Cannabis seeds
Clean water (distilled water is best)
Hydrogen Peroxide (H2O2) of known concentration (usually 3% when sold for household use)
Liquid measuring cup (1 cup / 250ml) and measuring spoon
Clean blunt-tipped tweezers
Small, sterile container with lid (e.g. glass jar or small plastic food container), one per each different type of seed you are germinating
Paper towels or coffee filters (unbleached paper is best)
Splash-proof label and pen
Thermometer
Tip: if you are going the “box of earth” route, an inexpensive stainless steel stem thermometer is a handy tool for observing the range of temperatures your seeds will experience in the intended location. Otherwise, use a laser or room thermometer to assess your options and determine a suitable spot.
For the “box of earth method,” partially fill a lidded tote or animal-safe box with sand or potting mix. Put it into the cool place, with your thermometer stuck into the “soil,” and note how close to 50°F (10°C) it is. Give your “earth” at least a few hours or a day to settle. Sitting on a cold cement or dirt floor, exposure to rain/snow, wind, direct sun, and other environmental factors may change its temperature. The purpose of burying the container is to use the “soil” as a buffer against rapid temperature changes, so keep it somewhere you expect conditions to stay fairly consistent for five days.
If you are going to use the box of earth method, please follow these guidelines.
Step 2, Day 1: Rinse seeds with a dilute solution of water and hydrogen peroxide
With clean, dry hands or tweezers, select the seeds you want to germinate and put reserved seeds back in their cool, dark, dry place. If starting seeds from more than one batch, make sure to keep different types separate and well-labeled throughout the process.
Mix 1/4 Cup (about 4 tablespoons, or 62.5ml) of 3% H2O2 with 1/2 Cup (125ml) distilled water. Put your seeds into the clean container, pour enough of this solution to cover them, and give it a swirl to ensure all the seeds get rinsed well.
Don’t skip this step: Cannabis seeds begin their germination journey by absorbing water. Because your seeds will be wet but not racing ahead at optimal growth temperature and going straight into germination, there is a higher risk of contamination. Since mold spores are everywhere, it is important to clean the seeds and be diligent about keeping everything they touch clean.
Step 3: Transfer seeds to a paper towel
In a clean environment and on a sanitary, flat surface, lay out a single sheet of clean paper towel. Dampen the paper towel with distilled water to the point of being very wet but with no pooling water. Ideally, your seeds will start to soak up this water over the next five days as they undergo the first phase of sprouting (known as imbibition).
Using sanitized tweezers, take the seeds from the container and arrange them on the wet paper towel 1 inch (about 2 cm) apart from one another and at least 2 inches (4 cm) from the edges of the paper. Gently roll or fold the paper towel to hold the seeds in place.
Step 4: Rinse again and put seeds into the clean container
Pour out the 1% H2O2 solution from your container. Use more of this solution to rinse it again.
Place your little parcel of wet seeds into the clean container and close it.
Step 5: Label your container
Note the Date, Time, Number, Name, and Type (e.g. feminized, autoflowering) of seeds being stratified. Put this information on the container and record it somewhere else (take a photo or start a new grow journal, for example).
Step 6: Use either the “box of earth” or “fridge method” to chill seeds
Box of Earth: half-bury the container with seeds and paper in the already-chilled “soil.”
Fridge: put your container of seeds into the fridge. If you’re using a food fridge, aim for a spot away from the vents and the light bulb.
Step 7: Prepare to transfer your seeds after five days
Set an alert on your phone, circle the date on the calendar, or otherwise make some kind of reminder to be ready to remove your seeds from the cold after five days.
Review options for germinating seeds in paper towels or sowing directly into your chosen substrate.
Step 8, Day 6: Remove seeds from stratification temperature and go ahead with your grow plan
After five days, collect the container from cold storage and use clean tweezers to transfer your seeds to their next home.
Note: If you germinate on paper towels, start with new material rather than keeping the seeds in the paper they’ve been sitting in for five days. This freshens the (warmer) water, provides a newly clean environment, and speeds up the switch between cold and warm.
Should seeds be stratified in the dark?
Cannabis seeds prefer a consistently dark environment in which to germinate. Use an opaque container or enough paper towel to significantly reduce their exposure to light (every time you open the fridge, for example) during the five days of stratification.
Final thoughts
Although stratification is not usually necessary for growing cannabis, folks can take advantage of its nature-mimicking processes to find greater success with older seeds and those that may have a more recent “wild” genetic in their heritage.
Experimenting with new techniques can enrich the experience of cultivating cannabis and help you become a more resilient grower. I encourage you to keep it simple and do what works, but always look for opportunities to learn!
Until next time, may you and your garden keep growing together, higher and higher!
Used sources for this article
Elias, Sabry G.; Wu, Yea-ching; and Stimpson, David C. (2020) “Seed Quality and Dormancy of Hemp (Cannabis sativa L.),” Journal of Agricultural Hemp Research: Vol. 2: Iss. 1, Article 2. https://digitalcommons.murraystate.edu/jahr/vol2/iss1/2
Draft Magazine, Beer Fridge Temp: A Guide to the Ideal Temperature for Storing Brews” https://draftmag.com/beer-fridge-temp/.
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