Overwintering Vegetables for Spring
There are two groups of vegetables that can be successfully overwintered into the spring: root vegetables and very cold hardy leafy greens.
Root vegetables should be planted early enough that they are a mature size for harvest by mid-November. As the ground begins to freeze, the root vegetables can be heavily mulched with straw to keep the soil from freezing. This method can provide an outdoor cold storage for the root vegetables. They can then be dug and used throughout the winter, as long as the soil is not frozen. Row covers can also assist with this goal.
Spinach, kale, some varieties of lettuce, and some specialty greens such as mache and claytonia are very cold tolerant and can be successfully overwintered with a couple layers of row cover. Depending on the winter and the amount of protection given, these vegetables may be harvestable for much of the winter or they may not have harvestable leaves until it is warmer with longer days in early March.
Row covers can be as simple as a thin, old sheet. However, for best success a lightweight, spun-bonded polypropylene fabric is recommended. This type of row cover can be found at local garden centers at certain times of year as well as through garden supply catalogs.
Row covers can be laid on the ground right over the plants to be protected or stretched over hoops. Because of frequent wind, it may be preferable to use hoops to prevent excessive rubbing of the row cover on the tops of the plants. As the weather gets colder, multiple layers of row cover can be used to provide a few more degrees of protection.
Row covers should be carefully secured on the edges so that the wind does not blow them off the plants. When using hoops, the tighter the row cover is stretched over the hoops, the better. The edges can be secured with bricks, cement block, heavy timbers, metal pins, or old milk jugs filled with water.
Kansas Climate Conditions for Winter Gardens
While most gardeners consider temperature to be the limiting factor in growing vegetables during the winter in Kansas, often it is the amount of sunlight that truly limits plant growth, because row covers and other techniques can keep very cold hardy plants alive throughout the coldest part of the winter.
From November 20th to January 20th there is less than 10 hours of daylight in south central Kansas. Most fall planted vegetables will exhibit extremely slow growth during this time period because there is not enough sunlight to encourage more growth. Any vegetables still alive during this timeframe should be either harvested and enjoyed or protected for growth to resume in the early spring. If you are planning to overwinter any vegetables or keep them past mid-November, you should plant early enough that they are mostly mature by mid-November.
In south central Kansas, our average low temperatures throughout the winter months show that we can successfully sustain many of our cool season vegetables throughout the winter.
Average Winter Months Low Temperatures:
November – 34 degrees
December – 24 degrees
January – 20 degrees
February – 25 degrees
March – 34 degrees
Although we can have winter low temperatures down below zero, many vegetables will be able to tolerate those short-term low temperatures if given adequate protection with row covers and mulch.
I know I’ve been harping on the fall gardening topics a lot and you’ve probably seen this video before, but just in case you haven’t…
While heat is the biggest challenge when planting your fall garden, by the end of fall there is the challenge of cold weather. In an average year, the first light frost will usually occur in mid- to late October, although sometimes the first frost occurs sometime in November. A light frost, where the low temperature dips down to 30-32 degrees will not damage fall vegetables. The cold may even improve the color and flavor of the vegetables. Most fall vegetables will tolerate temperatures down to 28 degrees without significant damage. In fact, allowing exposure to below-freezing temperatures will often allow the plants to better adapt to the cold, increasing their hardiness to later cold weather.
On a cold morning before the temperature is back above freezing, your plants may look wilted and permanently damaged. However, by the time the temperature is back above 32 degrees, they will recover and continue growing. It is important not to harvest these fall vegetables until the temperatures are back above 32 degrees and the plants have naturally recovered from the frost.
When low temperatures dip below the mid-to upper 20s, it will be necessary to protect semi-hardy fall vegetables with a light row cover to minimize damage on those vegetables. Some particularly hardy vegetables, such as spinach and kale, will not need extra protection until the low temperature is near 20 degrees.
It is usually best to remove the row covers during the day to allow the sun to warm the soil more efficiently when we are still getting intermittent frosts. As we get later into the year, leaving the row covers on may help create a mini “greenhouse effect” and prolong your growing season once we are regularly dipping into the 20s overnight.