Increasing native plant seed germination with cold stratification—it’s easy and fun!
As many native plants may not be commonly found in nurseries, you’ll need to locate seed and then determine the best method for growing it. Seed suppliers frequently include germination information, either on the seed packet itself or in their catalog, or these details can be found online. Growing plants from seed often requires working around “germination inhibition,” a strategy that plants have evolved to keep their seed from germinating before conditions are likely to be favorable for growth. Many of our native plants suppress germination until winter has passed. This type of germination inhibition can be easily overcome by giving the seed a period of “cold stratification” that mimics winter. It will also increase the rate of germination for some species.
Though necessary for successful germination of many species, cold stratification is not a difficult process. Basically you need to keep the seed cool or cold and moist for a number of weeks (this number varies by species). This can be accomplished in a variety of ways.
Cold Stratifying: Direct Sowing
Direct seeding in the area of your yard you’d like the plants in is about the least work-intensive method. Plant seeds in the fall and remember that all the seeds you plant may not germinate. Mark the area where they were planted and give it a gentle watering. Nature will complete the stratification process. Make sure you know what the seedling plants look like, or you may find yourself accidentally pulling them up while weeding. If you have a limited amount of seed, this method may be a bit too hit-or-miss for your liking—consider starting those seeds in pots.
Cold Stratifying: Pots
One of the most common methods used to cold-stratify is to plant the seeds in pots (be sure to label them in a weather-proof way) in December or January, water the pots, and then place them outdoors to be subjected to natural freezing and thawing. The north side of a building can help ensure they don’t see too wide of a variation in daily temperature swings, and it is a good idea to protect the pots from wind. Water as necessary through the winter so the planting mix doesn’t dry out. Topping off the pots with a half-inch layer of washed pea gravel or other fine rock mulch can help with this. If the seeds require light for germination, sprinkle them on top of the mulch. Eventually, come spring or summer, little seedlings will start to appear and are often ready to transplant by late summer. If no seedlings appear, leave the pots out for another round of winter weather; some seeds require two or more winters to overcome inhibition.
Cold Stratifying: Baggies
The other common method of cold stratification is to place the seeds in labeled plastic bags or other containers with moist perlite or vermiculite and stick them in the refrigerator for the requisite number of weeks (often 8 weeks, but it varies). Keep the containers closed to hold in moisture but periodically open to check in on the seed. When you start to see the white root tips emerge from the seeds, it’s time to gently pot them up in soil-less mix and place them under lights or in another location until they are big enough to transplant. For more information on growing these seeds indoors after they germinate, see the 2014 Winter issue of the Barnyards & Backyards: Rural Living in Wyoming magazine. This method generally allows you to get plants in the ground earlier than the outdoor pot method, but it is a bit more work and requires indoor space.
When carrying out either of these latter two methods, the germinating medium (soil-less mix, perlite, etc.) should be damp but not soggy. Seeds will rot if they sit in water for too long (they need some oxygen) or will become moldy.