Our greenhouse is full of flats of seedlings. Today is warm and sunny. At last! The soil is still too wet to till, but we are feeling more optimistic. The forecast still has possibilities of rain Tuesday night (only about 0.1″) and snow Friday night (less than an inch). Soon we will start moving flats from the greenhouse to our cold frames so the plants can harden off in preparation for transplanting in the raised beds. Usually we would have done this earlier, but it has been cold.
Our greenhouse construction is a masonry north wall and double-paned glass windows and insulated walls. Until last winter we didn’t use any additional heating, just the sun. But we now use an electric heater with the thermostat set so heat comes on if the temperature drops below 45F. We decided our seedlings are too precious to risk freezing them, and the weather is more extreme. We also put row cover over the seedling flats if the night temperature could fall below 18F outdoors. Once we have frost tender plants outside the plastic tent (which has a heat mat), we put row cover over those if the outdoor low temperature could be below 28F.
Our cold frames are built from loose set cinder blocks, higher on the north than the south. Following advice from Eliot Coleman in one of his older books, I think Four Season Harvest, we slope the soil in our cold frames. It’s higher at the north, by about 7 degrees, so the plants get better sun exposure.
In late summer, we dig compost into the soil in the frames, rake it up to the angle we want, and sow the spinach in mid-September.
The spinach grows in the ground in the cold frame all winter.When it gets cold enough, we cover with row cover. During cold spells we add lids which are wood frames with fiberglass glazing. For very cold nights we cover the cold frames with quilts which are made from reject scraps of the quilted hammocks Twin Oaks sells. Because they are made for outdoor use, the fabrics are very durable.
Here’s how we look after our cold frames in spring:
- If there is only spinach in the frames (no flats), use rowcover for temps >10F, rowcover and lids for <10F. No need to open and close every day, just remove or add lids if the weather is changing. See below about windy nights.
- When there are flats in the frames, at night, use the rowcover for temperatures 32-40F, rowcover+lids for 15-40F, rowcover+lids+quilts for temps below 15F. If winds are forecast to be more than 20mph, weight down the lids with wood. If you’re using the quilts, weight them down for winds more than 5mph.
- In the morning, if the temperature is over 20F, roll up the frame quilts if used.
- If the air temperature is over 50F, or over 45F and rapidly warming, open the cold frame lids if used, and remove any rowcover over flats, until it cools down again. Seedlings need to harden off, to prepare them to survive outdoors.
- Keep cabbage and broccoli over 40F, tomatoes, eggplant, celeriac over 45F, peppers over 50F.
- Flats of seedlings which have been up for a couple of weeks usually need to go to the cold frames for the last two weeks before their transplant date. Good to check about this though, and don’t do it if the weather is about to turn colder. Don’t put celery, eggplant, cucumbers, squash or hoophouse starts in the coldframe at all.
- If the soil surface is dry, run the drip irrigation for the spinach, or use the hose and sprayer for the flats. If frost is possible, disconnect the hose from the faucet and the sprayer head from the hose when finished, so that they do not freeze and burst at night. Please store the sprayer head in the blocks, don’t get sawdust or dirt in it, it’s a pain to clean. If the outdoor faucet is drained, carry water in cans from the greenhouse. Keep soil surfaces damp, not wet.
For a few years we have had trouble with voles eating the roots of the spinach. We found that if we replaced the spinach with flats of seedlings, they ate all the seedlings! So now, we leave the frames empty for a few days after clearing the spinach (any day now). Then we cover the soil with landscape fabric and put the flats on that. it saves our seedlings.