Flower Trialing Tech Joy Longfellow in her overwinter tunnel, spring 2020

Introduction to Overwintering Flowers

Johnny's Flower Trial Technician Joy Longfellow in her overwinter trial tunnel. Photo taken late May.

Methods + Recommendations from Johnny's Multiyear Overwinter Flower Trials

by Hillary Alger, Flower Product Manager & Joy Longfellow, Flower Trial Technician, Johnny's Selected Seeds

Many types of flowers can be started in late summer or fall, for overwintering. Overwintered crops tend to bloom about a month earlier than spring-planted field crops (with some variation between varieties), providing an abundance of high-quality blooms long before any field crops are ready for harvest.

Growers in different locations will experience different results. Some crops will overwinter successfully in your area, while others will not. Results can also vary from year to year. And, even if a particular flower crop overwinters successfully it may not provide a financially viable opportunity, whereas others will excel.

For all these reasons, we encourage you to start by conducting your own small-scale trials before scaling up your production plans. Then continue to experiment each year, to see what is feasible and learn more about the limits and possibilities of four-season cut-flower production on your farm.

Flower Crops to Overwinter — Results from Our Trials

The spreadsheet includes information on yield, rate of survival, and specific variety recommendations.

Top 5 Flowers for Overwintering in Tunnels

If you are new to overwintering, we recommend starting with one or more of our Top 5 Flowers for Overwintering in Tunnels:

  1. Sweet Pea
  2. Snapdragon
  3. Digitalis
  4. Dianthus
  5. Daucus

These 5 cut-flower crops have consistently performed well in our overwinter trials. They have proven the most reliable for survivability, high stem quality, productivity, and earliness, compared to field plantings.

25 Flower Crops to Overwinter

If you are ready to learn more about cut-flower crops for overwintering, read our Overwinter Flower Trials • Results by Crop. Covering all 25 of the cut-flower crops we have successfully overwintered in our Zone 5a unheated tunnel, this workbook provides information on yield, rate of survival, and specific variety recommendations. We set it up as a spreadsheet that you can download, save to your system, and put to work for your own planning and recordkeeping purposes.

Crop Planning & Timing: Last 10-Hour Day + First Hard Freeze

Last 10-Hour Day of the Year and First Hard Freeze Date are the two seasonal markers to note for your location.

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).

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 the 25 cut-flower crops detailed in our overwinter trial results.

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 and 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 use our Overwinter Flower Trials • Seeding Date Calculator, first approximate your Target Transplant Date by counting back 4 weeks from your Last 10-hour Day and/or Average Date of First Hard Freeze (below 28°F/ -2.2°C). Then, plug that Target Transplant Date into our calculator, and Your Earliest Seeding Date and Latest Seeding Date will display for each of the 25 crops.

Overwinter Flowers Using High & Low Tunnels

5 Fall Steps for Spring Success

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.
  1. Though some crops can be direct-sown, for tunnel plantings, we generally recommend sowing seeds in plug trays and transplanting the seedlings, to ensure a full stand in the precious real estate of a covered structure.
  2. Follow the standard Key Growing Information for germination and growing to transplant size.
  3. Transplant to prepared beds in a covered structure such as hoophouse or low tunnel.
  4. Allow about 1 month for seedlings to develop good root systems, before daylength drops below 10 hours/day and the first hard freeze takes place (usually the first week in November for us in Central Maine).
  5. Once outdoor temperatures consistently fall below freezing, install hoops and row cover over the plants within your covered structure, to add protection from frost as necessary. In Zone 5a, for example, we use AG-50, and recommend this heavy weight for other higher-latitude regions. Also take note of the more tender crops requiring a double layer of row cover "blankets" on our Overwinter Flower Trial • Results by Crop.

Typical Schedule for Our Farm

Our research farm is located in Central Maine, in Growing Zone 5a, Latitude 44.6°N.

Research has demonstrated how plants physiologically adapt to colder temperatures over time. A snapdragon planted in mid October in a tunnel in Maine will develop differently from one that is planted in the field in early May. Changes take place at a cellular level that allow the plant to withstand the cold temperatures and waning light. Plants are more likely to survive winter if they have a well-established root system. Scheduling the sowing date correctly allows the plant enough time to slowly acclimate to the cold, without expending too much energy into vegetative growth that will most likely die off in the winter.

A snapdragon plant that is full-grown going into the winter, in contrast, with a lot of vegetative (top) growth, is more likely to suffer tissue damage and death as a result of the cold winter temperatures. This dead tissue can become an entry point for pathogens during the long, cool, damp winter in the tunnel.

Our goal is to have well-rooted plants without excessive vegetation going into the winter. A deep root system will help keep the growing tips of the plants alive even if there is some leaf and tissue damage on outer foliage due to cold.
  • Late July–mid September
    • Sow seeds.
  • September
    • Prep beds in the tunnels.
    • Add organic amendments to soil as normal. Do not use any type of faster-acting fertilizer. You do not want much new top growth now. You want the plants to root out and acclimatize for winter, which takes transplants about 4 weeks.
  • October 7–15
    • Transplant into beds inside covered structure.
    • Seedlings should be about 3–4" tall and have 2 or 3 sets of true leaves at transplanting. If seedlings are too big, pinch off extra height.
    • We aim to have our overwinter tunnel planted by the time we plant our field tulips and daffodils in mid October.
    • Install low hoops to support floating row cover.
Overwintering results can be achieved with an unheated tunnel and/or a QuickHoops setup, to produce flowers to sell before shoppers have blooms in their own gardens. These crops pair well with spring bulbs, making sales for Mother's Day and beyond more diverse, more competitive for spring market bouquets and spring wedding flowers.
  • November–February
    • Cover crops with row cover when outside temperatures are forecast below 32°F / 0°C.
    • Keep seedlings watered, but do not overwater. You should not need to water after the ground has frozen. Plants may require water in the spring before the ground is thawed — consider in advance how your irrigation system will work in your tunnel if the ground is still frozen. In our trials, we have typically had to haul water to the tunnel 1–3 times in late February and early March, when plants start to grow but before the ground thaws.
    • Beware of heat or humidity build-up under covers. Be prepared to vent if needed.
  • Late February–March
    • Monitor for need to water — plants will begin growing as days lengthen and will likely need water before the ground thaws.
  • Late March or Early April
    • Remove row cover from beds as outdoor temperatures rise and plants grow.
    • We found some early crops (Centaurea, Agrostemma) outgrew the height of the low hoops before the rest of the crops in the tunnel. This, along with warming temperatures, determined when we stopped using row cover.
  • April–June
    • Harvest your blooms!

In our trials, most of the flower crops we overwintered in an unheated hoop house structure flowered about a month earlier and were of much higher quality than those produced in a typical unprotected spring-sown field planting.

But remember, this information is intended to give you a starting point. Your own method and timing may differ depending on your zone, latitude, and the severity of your winters.