If annual seedlings are growing in a tray that comes with a plastic top, or if you have a plastic mini-greenhouse, can you place them outside before the last frost date?? (Not plant them, but just covered/protected in their tray.) And if so, by how many weeks? (For either semi-hardy seedlings like larkspur/sweet pea or tender like cosmos/zinnia.) Toronto, Zone 6
Thank you for contacting the Toronto Master Gardeners. Seedlings can be moved outdoors in the daytime before the last frost. This is assuming that the daytime temperatures are above freezing and this is part of the ‘hardening off’ process for seedlings before they are planted in the ground. Placing seedlings in a covered, plastic seed tray outdoors in freezing or below freezing temperatures is similar to the idea of a cold frame. A key difference is that in our climate, cold frames are typically dug a few inches into the ground and covered with a clear plastic or glass lid that can be opened to provide ventilation. They trap daytime heat to warm the soil and moderate temperatures overnight. Cold frames can be insulated to extend their use into colder temperatures. A plastic tray that is above ground won’t hold the heat and will have more freeze/thaw cycles that will likely be damaging to young seedlings. To learn more about this from a previous Master Gardener post, click here Master Gardeners Cold Frames.
Having said this, there are people who practice Winter Sowing. This involves planting seeds outdoors in covered, ventilated containers during the winter (Jan./ Feb./ Mar.). Cold-hardy vegetables and flowers work best but most annuals are suitable and some people even grow warm season vegetables like tomatoes this way too. If a seed can germinate after being frozen, then winter sowing should work. The attached video by Robert Pavlis (Zone 5b) explains this process: winter sowing. With winter sowing, the seeds begin to germinate when the time is right (temperature, sun light) and don’t need to be hardened off as they are already outdoors. You may be able to use your growing tray or mini-greenhouse as a winter sowing container!
For further reading, Niki Jabbour is a Canadian garden writer and her book ‘The Year-Round Vegetable Gardener’ (Storey Publishing, 2011) also provides good information about extending the growing season. It is available through public libraries in Toronto.
Best of luck with your seedlings next year!
June 9, 2021