Outdoor Colorado Native Seed Stratification

| Colorado Native Plants, Propagation

By Jan Midgely

Rhus trilobata seeds.
Rhus trilobata seeds. Photo by Al Schneider courtesy of swcoloradowildflowers.com.

In nature, seeds from Colorado plants undergo large daily temperature swings over winter. Seeds of some species require a cold moist treatment (stratification) to germinate. Others do not, but will tolerate the cold. Temperature swings outdoors are quite different from a cold moist treatment in a refrigerator at a constant 38-40 ̊ F. For some species, the refrigerator works very well. For others outdoor exposure works better.

Let me suggest that you start a new holiday tradition. Sow your native plant seeds and put the containers outside and hope for a white winter. The more snow on top of the flats or pots, the better. Intentionally shovel a pile near the place you put the flats or pots for easy access. You can supplement the snow that falls naturally when it melts (and waters the seeds).

Let’s walk through the steps.

First, you need to choose the seeds that need a cold moist period. Many aster family plants, especially the ones we used to organize under the genus Aster, need no cold moist treatment. To learn if specific species of plants need some sort of special seed treatment to germinate, refer to the Wild Ones Front Range Germination Guide. Or search online by typing the botanical name followed by “seed.” For example, type Erigeron divergens seed in the search box.

Once you know which seeds you want to start, make a label with the date on it. Write in pencil for legibility over time.

Fill the containers with premoistened (just barely moist, not soaking) medium. A commercial germination mix or potting mix is desirable, because it will have adequate drainage and be pathogen and weed free. Save your compost for the vegetable garden. Lightly bounce the tray of cells or pots on the surface of the potting bench or press gently from above. Add more medium until the container is full to the top. Scrape off excess mix.

Erigonium jamesii seeds.
Erigonium jamesii seeds. Photo by Al Schneider courtesy of swcoloradowildflowers.com.

To sow the seeds, I like to cup seeds in the palm of my non-dominant hand and run seeds down the life line. I use a clam knife to advance and count the seeds. A clam knife is a 1/2”-1” spatula (available online for about $6). A small icing spatula or bread knife works also. Devices for sowing seeds are available online.

For larger seeds, place one to two seeds in a cell. Count out small seeds if possible. Division later disturbs the roots. For tiny seeds, sprinkle lightly over the top of the medium. For seeds in the aster family that have a fluffy pappus, sow pinches and press them into the medium. Wild-collected seeds often have lower fertility than commercial seeds and may require a more generous allotment per cell.

Cover the seeds more heavily than you might if you were spring sowing. Some materials for covering seeds include fine vermiculite or gravel (such as squeegee). Cover large seeds to a depth equal to the diameter of the seed. Some seeds need light to germinate and could be surface sown in the spring. When seeds are being cold treated outdoors over winter, I put a light cover over the seeds that have a light requirement. Exposure to snow and wind can be disturbing.

Gently water the cells/seeds, perhaps from the bottom, and label the tray or row with the name of the plant and the date.

Place the flats or pots on the north (or east) side of a building. Remove leaves from underneath or on top of the containers. Elevate the trays above the soil where slugs, snails, pill bugs and earwigs hide out. Use mesh trays, gravel, pavers or anything else you have on hand to raise the flats.

I protect the containers with 2-foot tall fencing and hardware cloth. My concerns are a young dog, rabbits and squirrels. You may have to deal with larger plant-eating critters.

In spring, place the flats in a location with more light. Depending on your elevation and temperature, you may need to water the flats in March. Some species germinate at very cool temperatures.

Seeds put out radicles underground that you usually don’t see. I had Winterfat (Krascheninnikovia lanata) seeds pushed out of the soil Dec. 21 (sown 11/23). I top dressed with some gravel smaller than bb shot because we expected a 45˚ temperature drop and high wind. No mama is out protecting these seeds in nature, but in nature the radicle is not constrained in 1.5” of soil.

The seeds will push up their first leaves when the temperature, moisture and light trigger them. This is one advantage to putting seedling flats outside. They germinate when conditions are optimal. Some seeds like to germinate at much colder temperatures than you might expect. Another advantage is time savings for the grower. Refrigerator stratification requires weekly inspection of the seeds and dripping in small amounts of water. If you have overwatered, the seeds may rot. Or they may extend radicles weeks before the last frost and then what do you do with them? All in all, shoveling a bit of extra snow on top of the flats or hand watering in unusually warm stretches beats weekly piddling with baggies and droppers.

Good luck! We expect to see and hear stories of many home-sown and home-grown native plants next year!

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