Introduction to Overwintering Flowers: Methods & Recommendations from the Multiyear Trials
By the Hensley's flower trial team
Guide to Overwintering Flowers
- Overwintering flowers: an introduction
- Picking crops to overwinter
- When to sow
Many flowers can be started in late summer or fall and overwintered. In our zone and latitude, many overwintered crops bloom roughly a month earlier than spring-planted field crops (varying by variety), giving plenty of top-quality stems well before the field is ready.
Of course, results will differ by location. Some crops will overwinter where you are and others won't, and results often change from year to year. And even a crop that overwinters well may not pay, while others may be very profitable.
That's why we suggest starting with small trials of your own before scaling up. Keep experimenting every year to see what works and to learn the limits and possibilities of four-season cut flower growing on your farm.
Preparing Your Unheated High Tunnel for Overwintering
Here's what to think about as you get your high tunnel ready for overwintering flowers.
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Mulch
We use white-on-black plastic mulch for weed and temperature control: the black underside blocks weeds and the white top keeps roots cooler. Cool roots matter during late-summer transplanting, when days are still long and warm, and again in spring as days lengthen and the tunnel heats up. Remember, these crops like it cool. We also put weed mats in the rows to keep weeds down.
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Plant Spacing
Spacing depends on your system. For most crops we plant 3 rows on a 36" bed with plants 6" apart. For very vigorous crops such as ammi and rudbeckia, we suggest 12" in the row. For quick, even spacing, lay Hortonova on the bed and use it as a planting grid.
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Frost Protection
We put hoops and heavyweight (AG-70) row cover over the plants.
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Trellising
Tunnel plantings grow lush, and plants are usually taller and more vigorous than in the field, so trellising and support matter even more.
For climbers like sweet peas, we run Hortonova vertically between T-posts, backed up with twine tied to the posts.
For upright crops, we lay Hortonova horizontally for support.
About tunnel growing: Most of our overwinter flower trials have been in unheated high tunnels with extra hoops and row cover inside. You can also overwinter in low tunnels or low hoops in the field, especially in milder climates where lighter protection is enough. In our limited tests with low tunnels and low hoops in Zone 5a, survival and quality were generally a little less reliable and crops bloomed a bit later than the same crops in high tunnels.
Creating an Irrigation Plan
Watering an unheated tunnel can be tricky, so plan ahead. We run 4 lines of drip irrigation on each 36" bed. That lets us water in transplants in late summer and get them well established in fall. Once freezing weather arrives in our Zone 5a tunnels (usually late November), we shut off the water, and we can't turn it back on until the ground thaws in late March or early April.
In winter, when days are under about 9 hours, plants mostly sit still: they grow little, transpire less and need little water. In early spring, as days lengthen, they start growing and need water, and for us that happens before the irrigation can be turned back on, so we haul water in barrels a couple of times in February and March depending on the weather. When you plan your system, think about how you'll get water to plants while the ground is still frozen in late winter and early spring.
Typical Schedule for a Zone 5a Farm
This sample schedule is for a Zone 5a farm at about 44–45°N latitude.
- Late July to mid-September
- Sow seed.
- September
- Prepare tunnel beds.
- Add organic amendments as usual, but no fast-acting fertilizer. You don't want lots of new top growth now; you want plants to root in and harden off for winter, which takes about 4 weeks after transplanting.
- Early to mid-October
- Transplant into beds under cover.
- Seedlings should be roughly 3–4" tall with 2 or 3 sets of true leaves. If they're too big, pinch them back.
- We aim to finish planting the overwinter tunnel by the time field tulips and daffodils go in, in mid-October.
- Set up low hoops to hold floating row cover.
- November to February
Our overwinter snapdragon trial in mid-January.Research shows that plants adapt physiologically to cold over time. A snapdragon planted in a tunnel in mid-October develops differently from one planted in the field in early May; changes at the cellular level let it withstand cold and low light. Plants with well-developed roots are more likely to survive winter. Sowing at the right time gives the plant enough time to adjust gradually to cold without putting too much energy into top growth that will probably die back in winter.
A snapdragon that goes into winter fully grown, with lots of top growth, is more likely to suffer tissue damage or die from the cold, and dead tissue can let diseases in during the long, cool, damp tunnel winter.
We want well-rooted plants without too much top growth going into winter. Deep roots help keep the growing tips alive even when outer leaves are damaged by cold.- Cover crops with row cover when outdoor lows are forecast below 32°F (0°C).
- Keep seedlings watered but not soggy. You shouldn't need to water once the ground freezes. Plants may need water in spring before the ground thaws, so plan ahead for how you'll water a tunnel with frozen ground. In our trials we've usually had to haul water 1–3 times in late February and early March, when plants start growing but the ground is still frozen.
- Watch for heat and humidity building up under covers, and vent when needed.
- Late February to March
- Check whether plants need water; they'll start growing as days lengthen and will probably need water before the ground thaws.
- Late March or early April
- Take row cover off as it warms up outside and the plants grow.
- Some early crops (centaurea, agrostemma) outgrew the low hoops before the rest of the tunnel, which, along with rising temperatures, told us when to stop using row cover.
- April to June
- Harvest!
In our trials, most flowers overwintered in an unheated hoophouse bloomed about a month earlier and were much higher quality than typical spring-sown, unprotected field plantings.
Remember, this is only a starting point. Your methods and timing may differ depending on your zone, latitude and how harsh your winters are.
5 Fall Steps for Spring Success
- Some crops can be direct-sown, but in a tunnel we usually recommend sowing in plug trays and transplanting to guarantee a full stand in valuable covered space.
- Follow the standard growing information for germinating seed and growing it to transplant size.
- Transplant into prepared beds in a covered structure such as a hoophouse.
- Give seedlings about a month to root in before days fall below 10 hours and the first hard freeze arrives (for us, usually the first week of November). For more on finding your sowing and transplant dates, read When to sow flowers for overwintering.
- Once outdoor temperatures stay below freezing, put hoops and row cover over plants inside the structure for added frost protection. In Zone 5a we use AG-70 and recommend that heavy weight for other northern areas too. Check which more tender crops need a double layer of row cover in our overwinter flower trial results by crop spreadsheet.
Learn More
- Browse our flower varieties suited to overwintering
- Picking flower crops to overwinter: trial results
- When to sow flowers for overwintering
- Overwintering flowers webinar: a longer season with cold-hardy annuals • ALL RESOURCES
- Overwinter flower trial results by crop • XLSX
- Overwinter flower seeding date calculator • XLSX
- USDA hardiness zone map • MAP
