9 Seed Cold Stratification Mistakes: What NOT To Do
Several native annuals and perennials need to be sown directly into the soil in fall and winter. This process is called cold stratification, and it requires timing and preparation. In this article, gardening expert Matt Dursum shows you common seed cold stratification mistakes and how to avoid them.
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If you’ve ever sown your seeds in a northern climate garden, you’re probably familiar with the term cold stratification. Some varieties are native to cold climates where severe winters bring temperatures far below zero.
To adapt, these species use the extreme environment to germinate at the perfect time. They stay buried in the icy ground and snow until spring. When the snow melts and the sun shines, they begin germinating.
If you live in warm climates, you can mimic this process by putting cold-hardy seeds in the fridge for a few weeks. This lets you grow a diverse array of annuals and perennials in your warm-climate garden.
Regardless of where you live, you’ll need to cold-stratify several seeds to get them to germinate. The process is straightforward, but you’ll still need to pay attention to what you’re doing. Below are 9 cold stratification mistakes to avoid this winter.
Mistake 1: Sowing Too Late
You may have second thoughts about sowing outside in the freezing air. But if you’re planting cold-climate annuals or perennials, you’ll have to trust their biological process. Getting them exposed to frozen temperatures too late can be detrimental and wasteful.
These hardy species need several weeks in the blustery weather. They have protective outer layers that shield them from the elements. The frigid weather helps weaken their hard outer shells, allowing them to break out of dormancy once the warm weather returns.
If you sow your hardy annuals and perennials too late or you’re experiencing warmer-than-average temperatures this winter, don’t worry. You can mimic cold stratification in the fridge instead. Your fridge has temperatures around 35-45°F (2-7°C), which should be enough to stratify. Place them in a moist paper towel, media, or substrate and after a few weeks, they’ll be ready for spring planting.
Mistake 2: Cold Stratifying the Wrong Species
If you sow tropical or sub-tropical plants like jalapenos in freezing temperatures, they won’t germinate. These plants are native to warm areas where freezing temperatures are rare. They never developed a process to break out of dormancy in the cold.
Most of the crops we love are from warmer climates. It is a mistake to expose these seeds to cold stratification because they’ll never germinate. You’ll end up wasting entire seed packets. Even cool weather-loving vegetables, such as spinach and broccoli, won’t germinate in severe winter temperatures.
When you buy your seeds, read the package carefully to see if they need cold stratification. Below are common seeds that germinate in the cold:
- Milkweed
- Black-eyed Susan
- Catmint
- Evening Primrose
- Goldenrod
Mistake 3: Sowing Too Early
Sowing your hardy varieties too early can be more problematic than sowing too late. This could expose them to warm temperatures before they’ve had their necessary time in the chilled temperatures. Warm temperatures could make them germinate too early. When this happens, any shock of sub-zero weather will kill your delicate seedlings.
Some native perennials and hardy crops need to be sowed by late fall through early winter. This gives them enough time in the frigid weather before germinating in spring. In extreme northern climates, this could be as early as October. In temperate regions with milder winters, you may have to wait until December or January before sowing.
Mistake 4: Letting Seeds Dry in the Fridge
If you’re stratifying in the fridge, you’ll have to keep the moisture level high but not saturated. The best way to do this is by placing them on a moist paper towel, or in a bag with moist growing media. If you forget this step, your seeds may dry out and fail to germinate.
Spread them out so they’re not overcrowding each other. Place the moist towel or substrate in the fridge and check on it every week. If it dries up, give it a misting of water.
Moisture helps soften the thick outer layer protecting the seeds. It also keeps them from drying out. Most hardy species won’t germinate unless they’re in a frigid, moist environment.
Mistake 5: Letting the Substrate Grow Mold
If you’re preparing your seeds for cold stratification and germination in your fridge, watch out for mold-forming mistakes. Any material you choose can attract mold, especially if your fridge temperatures go above 45°F (7°C).
Some molds can grow in temperatures above 41°F. If your fridge is on the warmer end, you may see mold spores start to grow. If left unchecked, the mold will eventually infect your seeds before they germinate.
To prevent this, keep your fridge at a low temperature. Check on them often and replace the substrate if you see any signs of mold spores.
Mistake 6: Overcrowding
Some species can flourish if you sow them directly in the soil, while others won’t. These varieties need to grow spaced apart. They’ll compete for nutrients and water and experience stunted growth.
Check your seed packets for exact spacing requirements. These can be anything from a few inches to simply scattering on the soil surface. If you see seedlings growing in large clusters and competing with each other, you can often thin them out by hand. This will encourage the stronger seedlings to take root.
Read your packets for sowing directions and carefully distribute your seeds on the soil or snow. Take your time to make sure they’re evenly distributed. This gives them enough space to share resources without overcrowding.
Mistake 7: Not Understanding Your Climate
When you sow depends on the climate of where you live. It also depends on the year. Typically, the longer your winter is, the more time you have for cold stratification, which means it is a mistake to underestimate the intensity of the cool season. Warmer climates may not have as many freezing days, so you’ll have to give them as much time as you can in the frigid air.
If you only experience a few sub-freezing days a year, you’ll probably have to cold-stratify in the fridge before the spring planting season. If you experience hard winters, you’ll have plenty of freezing days to work with.
Get to know your average winter weather. Take note of the average times of year you experience the first and last freezes. Check your seed packets to see when you should sow in your particular climate. Each region in the US has a plant hardiness zone, so become familiar with your zone and what can grow there.
Mistake 8: Freezing Your Seeds
Just because these varieties need to be exposed to low temperatures doesn’t mean they need to be frozen. When northern-climate perennials scatter their seeds in their native environment, heavy layers of snow form over the fallen seeds. Snow often keeps the soil temperature above freezing and moist.
Some gardeners try to cheat the natural process by freezing their seeds in ice, but this is a big mistake. The water will moisten the protective exterior. When the water freezes, it expands, forming cracks and splits. This will damage their plant cells and eventually kill them.
Cold storage in temperatures between 35 and 40°F (2-4°C) is a good way to preserve your seeds for years. To do this properly, you don’t want to freeze them or expose them to moisture. Simply keep the dry seeds in cold storage until they’re ready for sowing in spring.
Mistake 9: Changing Temperature Exposure
When you sow outside in the late fall or winter, make sure they’re in a place that stays at a consistently frigid temperature. Sometimes, a heater, car exhaust, or garage entrance can warm up the soil just enough to trick your perennials into germination.
If the snow or ice melts and your perennials and hardy annuals suddenly experience hot temperatures, they can start germinating. If they start germinating too early, any sudden freeze or temperature drop can kill them. This can be especially risky in Northern climates, where late-season frosts are frequent.
Avoid this by sowing away from any sources of heat. Let nature do its magic and keep them in a consistently chilled environment until spring.
Key Takeaways
Many perennials and cold-climate annuals need cold stratification to germinate. This process exposes them to cold temperatures that prepare them for their dormancy cycle. Once the temperatures warm up, they’ll start to germinate.
If your seed packet instructions say to cold stratify, you have a couple of options. If you live in northern climates with icy winters, try sowing directly outdoors. If you’re in a climate that doesn’t experience freezes, sow in a moist substrate and store them in your fridge for the period of time they need to break dormancy.
If you don’t make the mistakes mentioned above, you’ll enjoy healthy plants in the summer. Hardy perennials and annuals can be some of the most beautiful flowers and ornamental plants in your garden.
