Cold Frames and Winter Sowing
Use cold frames and milk-jug winter sowing for cool-season crops, tied to your zone calendar, not invented January dates.
Published ยท Reviewed by Blake Peavy
cold frame winter sowing winter
Cold Frames and Winter Sowing
A cold frame is a low glazed box that traps sun, blocks wind, and keeps soil a few degrees warmer than open ground. Winter sowing uses outdoor containers (often translucent milk jugs) with vents so hardy seed sprouts when the season is ready, not on a fixed indoor countdown.
Neither method replaces your frost date. Set last and first frost in My Garden, then check What to plant now, the Winter hub, and a zone calendar such as zone 6.
What Cold Frames Do
- Shelter seedlings and salad greens from wind and light freezes.
- Warm soil enough for cool crops whose catalog mins sit near 35-45 F (spinach 35 F; lettuce, kale, arugula about 40 F; broccoli and cabbage about 45 F).
- Need venting on sunny days so plants do not cook. Prop the lid; close before hard freezes.
- Pair with frost covers when a hard freeze is forecast.
Winter Sowing Cool Crops
Winter sowing fits crops that tolerate cold germination and outdoor swings. Tender crops such as tomatoes go in jugs only late, about 6 to 3 weeks before last frost.
Catalog cool crops that fit frames or jug sowing:
- Greens: lettuce, spinach, kale, arugula, Swiss chard
- Brassicas: broccoli, cabbage, collard greens
- Fast roots: radish, turnip, beet
- Pea where your spring window is open
Milk-jug method in brief: clean jug, drain holes, sow at catalog depth in moist mix, tape shut with a top vent, label, and set outside in light. Open fully as seedlings fill the jug and nights ease, then harden like any transplant. See How to harden off seedlings.
Full steps are in How to winter sow in milk jugs. For sow dates by zone, use the Winter Sowing Desk. It counts four sowing waves back from your last spring frost: perennials that need cold, hardy flowers, cool crops, then tomatoes and other tender crops.
Timing Without Invented Dates
Do not use a national "sow in January" rule. Use:
- Your frost settings in My Garden (first frost date).
- Zone fall and spring catalog windows on planting calendars (warm zones often still sow greens into December; many zones reopen cool crops in late winter. See live Dec, Feb zone guides).
- Soil readiness: if the mix is frozen solid day after day, wait; winter sowing is patient.
For full indoor heat-mat crops (tomato, pepper), prefer indoor seed starting timed to last frost via seed starting.
Safety Notes
- Vent frames and jugs on bright days.
- Water sparingly in deep cold; overwater in closed containers causes rot.
- Weight lids against wind; secure jugs so they do not blow away.
- Transplant only after plants are sturdy, and after harden-off if they lived in a sealed jug.
Next Steps
Pick one cool crop from the list, match it to your zone calendar, and start a frame or jug batch. Continue tray work under lights when warm-season timing hits. Cross-links: indoor seed starting, harden-off, cover crops by zone, Winter.
A jug does not reliably protect emerged seedlings from frost. Move tender seedlings to a frost-free place when a freeze is forecast. The listed windows are examples, not a guarantee of safe outdoor conditions.