Guide to Overwintering Flowers

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1. Introduction to Overwintering Flowers
2. Choosing Flower Crops
3. When to Start Seeds
   • Target Transplanting Date
   • Seeding Dates
   • Learn More

Crop planning for overwintered plantings can be a challenge. The timeline can seem counterintuitive, because seed-starting for many overwintering varieties takes place amidst the bustle of summer harvest (July and August for Zones 5a–5b).

Identify Your Target Transplanting Date

This photo of a recently planted overwinter flower trial in one of our tunnels was taken October 24.

Transplanting dates for our overwinter flower trials roughly coincide with fall flower bulb and garlic planting on our farm, which also serves as a good seasonal reminder for planning purposes.

Location, because it governs light and temperature, is key to establishing your timeline. The two seasonal markers to note in your location are: the last 10-hour day of the year the average date of the first hard freeze (below 28°F / -2.2°C). This is when temperatures will start to routinely drop below freezing. Decreasing daylength and temperatures have a compounding effect, causing plant growth to slow to a halt.

Here in Maine, daylength dips below 10 hours per day during the first week of November. So on our Research Farm in Albion, we target planting dates between October 7–15 for our overwinter tunnel crops, to allow about 4 weeks for seedlings to develop roots and become established before winter sets in.

To identify your target transplanting window:

1. Use an app or website to identify the last 10-hour day of the year and the average date of the first hard freeze for your location.
2. Count backwards by about 4 weeks from your last 10-hour day; this is your transplanting window.
3. Check your transplanting window against your hard freeze date. If your transplanting window falls several weeks past the point when you expect regular freezing temperatures, then move your transplanting window up, accordingly.