Cold Stratification for Seeds: How Does it Work?

Some plant packages suggest planting seeds in the fall so they can sprout in spring. It may seem a little weird if you’re unfamiliar with seed stratification. Won’t the cold weather kill the plant? While some will die in the cold, others need cold exposure to start their germination process once the weather warms up again.

Seed stratification may sound impossible in a hot climate with warm winters. You’re not out of luck; the stratification process can be mimicked in a cool environment like your fridge. This is helpful if you buy seeds towards the end of winter or never get cold temperatures.

Stratification is a simple process, but it takes a little knowledge to ensure you do it right. If you don’t let seeds chill long enough or let them chill too long, you risk low or no germination. There’s also the hurdle of learning which types do and don’t need this cold process. I’ll help you figure everything out so you can try it yourself. You better start clearing some space in your fridge!

The Long Answer

It sounds straightforward, but you should know many little things before you start stratifying. Let’s look at the details to set yourself up for success.

What Is Seed Stratification?

Seed stratification isn’t a nifty gardening hack; it’s a survival mechanism. In nature, seeds must protect themselves from cold weather to survive and carry on their species. Germinating too early will kill some plants, which is why it’s so important that they have a process in place to wait for the perfect timing.

Seeds stay buried beneath the snow in cold climates for several months. Early germination would be a certain death. But when the snow melts, and the temperatures warm up, the seed knows it’s time to open up and establish its roots.

You can play as Mother Earth and duplicate this process at home in your refrigerator. Gardeners love to collect plants from around the world, meaning that your plants might not have the ideal conditions to develop in your area. Letting seeds spend a little while in your fridge opens the doors for growing many exotic plants you wouldn’t normally get to enjoy in your garden.

In most cases, the temperature of the soil remains warmer over the winter months than the ambient air temperature. Interestingly, snow is also an insulator in a similar way to mulch; a nice heavy load of snow can keep the warmth under the soil at the right level for your seed. In most cases, soil temps of 35-40 degrees Fahrenheit are all that’s necessary, so long as they’re consistent throughout the winter.

How to Know If a Seed Needs Stratification

The best way to know if a seed needs to be stratified is by looking it up from reliable sources, but there are a few ways to guess if a seed needs it or not.

The first sign is if you tried to plant the seed in the spring and no germination occurred. If all the conditions were right—proper lighting (or lack thereof), water, sun, and soil—and all you received was a disappointment, then you’re probably missing the seed’s required cooling stage.

Seed Coat Thickness

Look at the seed and notice its coat. Break it open if you have extras (for the sake of education). Does it have a thin or thick seed coat? Thin coats will germinate easily because the tender seedling doesn’t have to break through much, but even the tiny seeds with thin seed coats may still need a period of cooler weather. Thick coats need constant damp conditions to allow moisture to penetrate the heavier layer and may even need scarification before spending some time in the fridge.

Seed coat thickness isn’t a universal method of telling if a seed needs stratification; some thick-coated seeds, like sunflowers, don’t need the cold at all and should not be cold-stratified. Most warm-weather plants also don’t require cold stratification. If your seeds say to plant them after your final frost date, assume they don’t require the cold and plant them as directed on the seed packet!

Many annual wildflowers, like Purple Coneflower and Black-Eyed Susan, need cool temperatures to jumpstart their spring growth, but you probably wouldn’t be able to guess it when comparing their seeds to something obvious, like a pecan.

Generally speaking, herbaceous perennials and woody plants typically need to be stratified. Of course, there are always outliers, so researching or following the directions on your seed packet is your safest bet.

Stratification Methods

Cold stratification requires cold temperatures, so you’ll need a refrigerator. However, if you live in an area with cold winters, you can simply direct sow in the fall. The seeds will cold-stratify on their own and will pop up in the spring.

In the fridge, temperatures should be in the upper 30s or low 40s Fahrenheit and should never drop below freezing. Modern fridges typically have a thermometer built into them, but you can place one inside the fridge if needed. If the temperature gets too cold or warm, it could mess up the process.

The fridge isn’t the only method. If you’re stratifying on a large scale or don’t have enough room in the fridge, you’ll likely need another method. A cooler will do just fine, provided you can keep the temperature right and keep the seed from drowning in melted water. A rolling cooler can be kept cold with ice packs, but this may be difficult since you’ll need to replace the ice packs frequently, requiring you to open the lid and let the cold out.

You can also use cold rooms like a cellar, walk-in cooler, or chilly basement, provided you can keep the rooms at the optimal temperature. If you live in a cold area, burying the seeds in a shady area can help, too.

Do not put your seeds in the freezer unless you want freezer-burned seeds! Most home freezers are opened and closed far too often. This allows heat into the freezer, and seeds thaw quickly; this constant freeze-thaw-freeze-again cycle often results in a die-off of seeds.

Simulated Stratification

To simulate winter in the refrigerator, you must choose which method to use. There are a few different options that work well:

• Plant seeds in pre-dampened seed starting mix in a pot or cell tray, then loosely drape a piece of plastic wrap over the soil and put the container into the refrigerator,
• Dampen fine sand until it clumps together, then blend your seeds into the damp sand and put it in a container or baggie in the refrigerator (but do not fully close the container or baggie),
• Dampen peat moss or coconut coir until it can form a ball, then blend your seeds through it and put it in a container or baggie in the refrigerator (but do not fully close the container or baggie),
• Dampen a paper towel and wring it out so it no longer drips, then lay your seeds on it in a single layer and place a dry paper towel on top. Roll it up into a tube before putting it in a baggie or container in the refrigerator (but do not fully close the container or baggie).

Notice a trend in these methods? In all these methods, the medium you’re putting your seeds into is damp to the touch – not soggy or dripping, just damp. This allows moisture to penetrate the seed coat gradually. In most cases, they also use a container or a baggie, but one that is not sealed; this reduces the risk of mold forming in an anaerobic environment. Your seeds need to breathe, too!

How long does it take?

Label your prepared seeds with the date and species, then tuck them into the refrigerator. Be sure to know how long they need to be exposed to cold temperatures; in many cases, a month is enough, but some seeds native to colder climates may take as much as three months. Look at the information on the seed packet for details. More is not always better; overdoing it can damage your seeds!

While the Seed Is Stratifying

Stratification is a waiting game, but you do have some minimal maintenance chores to tend to while your seeds nap in the fridge. It’s important that your cold stratification medium stays moist to the touch and mold-free, so you’ll need to check it periodically. Once a week should suffice unless you suspect there are problems.

If it seems too dry, add enough water to make it moist, but it shouldn’t be dripping or allowing water to pool in the container. Too much moisture can promote mold development, which will kill your seeds. Mold is most common in paper towel-based cold stratification as you’ve got multiple layers of towel wrapped up; the center may not breathe as readily as the exterior. To avert this crisis, try one of the other methods suggested above.

Ensure the temperature is properly maintained. If your location isn’t cool enough, try to find a new place or supplement the temperature with ice packs. Keep a thermometer nearby so you can get an accurate reading. Ideally, you want to check the temperature of the medium that you’ve got your seeds in, -not- the ambient air temperature; aim for a range between 35-40 degrees Fahrenheit.

Finally, you’ll need to check and ensure you don’t see any signs of germination. Remember, the key things that spur germination are moisture and warmth; if your medium ends up warmer than it should, or your seed type is a cold-weather variety, you can have early germination.

If you see signs of life (which is easier to spot in a sand, coir, or peat moss medium than in wrapped-up layers of paper towel), immediately remove the young plants and get them planted; your cold stratification is complete!

When It’s Time to Plant the Seed

For most seeds, it will be time to plant right as the temperatures warm up in early spring. If you’re planting indoors, the time of year won’t matter as most of us keep our homes at the right temperature range; as soon as they come out of cool storage, they are likely to sprout very quickly.

While you should not see sprouts when you take them out of storage, there is always the possibility that a few may have. You can plant them, but be very gentle with sprouted seeds as the new root is likely to be very fragile, and the leaves may be equally fragile. Ones that sprout in the refrigerator likely had inconsistent cold temperatures; try to keep it consistently between that 35 to 40-degree range and avoid opening the refrigerator too often during the stratification process.

If you don’t see sprouts, plant your seeds immediately after removing them from cold storage and monitor them for signs of growth, then treat them as you would any other seedling plants.