How to Overwinter Fall Crops

How to Overwinter Fall Crops

Even in zone 4B in central Wisconsin, I have managed to get certain vegetables to overwinter. One important point is that there is a difference between overwintering vegetables and actively growing them. Temperature is not the only consideration for plants to grow. The total amount of sunlight is also a big concern. The “Persephone Period” sometimes called “Persephone Days” is the time period when days have 10 or fewer hours of daylight. Most plants need a minimum of 10 hours or more of sunlight for vegetative growth. During the “Persephone Period” plant growth slows or completely stops even if temperatures remain above freezing. For me, November 5th is the first day with less than 10 hours of sunlight and I will not go above 10 hours of daylight until February 5th. Most of this time it is too cold to grow anything anyway, but even if I had a heated greenhouse I could not grow much unless I also gave the plants supplemental light. You can check out this site  to find your day lengths.

​So why bother overwintering vegetables if they are not going to grow much anyway? Basically, overwintering allows you to have fresh vegetables either in the middle of winter or earlier in spring than you otherwise would. Additionally, many root vegetables like parsnips and carrots taste better after a period of cold weather. I have been able to successfully overwinter spinach, kale, leeks, parsnips, and carrots, although the spinach and kale do look a little rough in the spring, but fresh growth usually starts as soon as the daylight increases and temperatures warm. One important point is that you want to harvest your vegetables fairly early in the spring or there is a risk of them going to seed. Once they start going to seed the root vegetables will be woody and the greens can get bitter.

​The two things I do to overwinter my vegetables are frost covers and mulching. I put layers of mulch, usually leaves, over my vegetables, I try for about a foot thick, but I have read if you go even deeper, 3 to 4 feet thick, you can dig root vegetables all winter long. We usually get too much snow to easily clear away the snow and leaves to dig so I generally just leave them there all winter and uncover in the spring. The second thing I use is frost covers. I started buying higher quality frost covers from A. M. Leonard or Johnny’s Selected Seeds which last longer than cheaper frost covers. Regardless, do plan to replace your frost covers occasionally as they get holes or tear. You can buy frost in various thicknesses which can give you a degree or two of protection up to 5-10 degrees. I also use frost covers in the fall to protect my crops from any early hard frosts. If you are still getting enough sunlight to grow crops you may not want frost covers too thick because these also block sunlight. Each frost cover description should give you the thickness, the degrees of protection and how much sunlight is blocked. So, it is a balance between enough frost protection and enough sunlight. For overwintering, I lay the frost covers over my mulched vegetables and weigh down the sides to keep cold air out and to keep them from blowing away.

In the spring, once temperatures have warmed up enough, I pull away all the mulch but leave my frost covers over my vegetables to provide some protection. As soon as the ground thaws you can dig up your root vegetables. Greens and onions can be eaten as soon as they start to perk up, grow a little, and basically look edible. Also, as long as the ground is thawed make sure to water if needed. Vegetables should be well watered before covering them up, but you do not want standing water which could allow them to rot. Many plants die over the winter not from the cold but from drying out in the cold winter air and winds. Frost covers also help keep the cold wind from penetrating into your plants.

The last consideration is what varieties of vegetables to choose. I have had good luck with Vates kale but I have had Red Russian kale die at 17°F. Blue Solaise leeks routinely overwinter for me although a few may rot. I have gotten Winter Giant Spinach to overwinter as well as Hollow Crown parsnips and any fall/winter carrot variety such as Oxheart, Kuroda, and Chantenay.

Resources and References

1. Dake, Helen. The Persephone Period. Napa Master Gardener Column.  https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=28043