Starting seeds that need special treatment
Some seeds with a hard outer coat need a nudge to germinate. In nature, they would be soaked by rain, heated by fire, kept cold over winter or eaten and excreted by animals. To start them indoors, you need to simulate these processes before planting the seeds in clean containers of potting mix. The treatments are scarifying and stratifying. Here's how they both work.
1. Scarifying seeds: Breaking through the seed coat to allow moisture to enter.
Methods of scarifying include:
- Nicking the seed coat with a razor blade or nail clippers, abrading it lightly with sandpaper, or pricking it with a needle. The key is not to nick, scrape or prick too deeply and damage the embryo.
- Soaking seeds in warm water for at least 24 hours softens the seed coat or removes chemicals that inhibit germination. Seeds such as beets, carrots, parsley and spinach benefit from being soaked in warm water for at least 24 hours before planting.
- Some seeds, such as New Jersey tea (Ceanothus americanus), black locust (Robinia pseudoacacia) and other legumes, are best covered with hot but not boiling water and left to cool for 12 to 24 hours. If scarified properly, the seeds will either swell or sink.
2. Stratifying seeds: Exposing the seeds to a period of cold but not freezing temperatures.
Moist stratification: To simulate a snowy winter, put seeds in a plastic bag with an equal amount of damp sawdust, sifted peat moss, or horticultural sand, mixed with a little vermiculite. Set the bag in a warm spot for a couple of days to allow the seeds to absorb water, then put it in the fridge at 3° to 4°C. Perennial seeds need at least three or four weeks, while tree/shrub seeds may need up to six months.
Dry stratification: Put seeds in an airtight container in the freezer for 30 to 90 days.
Many seeds need a combination of treatments: black-eyed Susan (Rudbeckia spp. and cvs.) and columbine (Aquilegia spp. and cvs.), for example, are best pre-soaked and then stratified; false indigo (Baptisia spp. and cvs.) and Eastern redbud (Cercis canadensis and cvs.) benefit from being abraded and stratified, while canna lilies need to be nicked and soaked. Every seed has its preferred means of germination so do some homework to make sure you’re matching your method correctly.