Flower Farming

Overwintered flower plants in a high tunnel

When to Start Seeds for Overwintered Flowers

Overwintering flower crops in a high tunnel on the Johnny's Research Farm.

By Hillary Alger, Flower Product Manager, and Joy Longfellow, Flower Trial Technician, Johnny's Selected Seeds

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

A recently planted overwinter flower trial tunnel. Photo taken Oct 24. 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.

Calculate Your Seeding Dates

Overwinter Flower Trials Seeding Date Calculator
Download our Overwinter Flower Trials • Seeding Date Calculator to schedule your sowing and transplanting dates.

Now that you know your target transplanting date, you can now calculate the seeding dates for the various crops you intend to overwinter.

To make timing more straightforward, we developed a Seeding Date Calculator based on findings from our multiyear overwinter flower trials. You can use the calculator to determine the best seeding date range in your area for over 25 cut-flower crops detailed in our overwinter trial results.

To use our Overwinter Flower Trials • Seeding Date Calculator, plug your Target Transplant Date into our calculator, and Your Earliest Seeding Date and Latest Seeding Date will display for each of the 25+ crops.