Cold frames: What they are and why you should use them

Have you ever grown so many carrots, lettuce, spinach, onions or beets that you can’t eat them all by the time cold weather arrives? Don’t want to freeze, can or pickle? Or maybe you haven’t had enough of your fresh, home-grown produce. Well, a cold frame might be an answer to consider.

A cold frame is a low-tech alternative to a greenhouse, with a lower profile (in case your HOA is watching) and smaller footprint. While not without some effort, making and using a cold frame can harness the benefits of solar energy to keep mature (or almost mature) vegetables in a dormant state until you decide to harvest them. Baby plants will not grow – or not grow very much – but mature plants will go into their own type of hibernation until you’re ready to use them.

By Susan Perry
Colorado State University Extension Master Gardener in Larimer County

Simply, a cold frame is a wooden frame you set on the ground around your cold-hardy vegetables. Think of it as a small, 12” high greenhouse with wooden sides, no bottom and a cover that allows sunlight to penetrate. During the day, the sun warms the air and ground inside, even on cloudy days. In fact, often on sunny winter days, it’s necessary to vent (or open) the cover so the inside doesn’t become too hot. The first year I tried this, a 50 degree sunny January day successfully fried my lettuce by 11 a.m. The temperature in the tiny hoop house registered 90 degrees!

In the afternoon, shortly before the sun goes down, put the glass back on so that the inside of the box stays warm during the night from heat retained by the ground. The cold frame creates a micro-climate in which the plants can survive cold nights or extreme cold spells.

To take advantage of daily solar gain, make the north end of the cold frame taller than the south end. The east and west sides will slope from north to south. If you’re not the handy type or don’t know someone handy with a saw, then skip the sloping – a wooden box with glass or clear plastic on top will work just fine. While not ideal, in future years you can make enhancements. The important thing is that the box doesn’t let air in unless you vent it, so things need to fit together well. The cover can be an old glass storm window, well anchored plastic sheeting, or other more rigid clear plastic. The first year I tried this I made an 18” high hoop from chicken wire, covered it with clear plastic, and clothes-pinned the plastic to the chicken wire. During really cold spells, I threw a blanket on top. All was well until that fateful, sunny January day mentioned above.

There are many resources on the Internet that describe how to make a simple cold frame. Factsheet 7.409 from CSU Extension (www.ext.colostate.edu) briefly describes how to construct a cold frame to start plants from seed. (Another benefit of cold frames is they are a great way to get an early start on your plants in the spring.) Other resources are available at our local libraries.

Often these resources recommend orienting the cold frame in one direction or another (north-south, east-west). In my opinion, you need to make it work for your garden, regardless of what the experts say. In our case, our beds are long from north to south and narrow from east to west and so my cold frames are oriented to work within this constraint — much easier than trying to re-orient the entire garden. However, I refer you to these resources because they can describe the construction details for cold frames in far more detail than can be addressed here.

Last year, as an experiment, we used a cold frame to prevent the ground from freezing around the several hundred carrots we had in October. We quickly cobbled together one cold frame using stacking, hinged wood boxes from a local recycler in Timnath, lined them with Styrofoam, and covered them with two old aluminum storm windows. We enjoyed carrots much of the winter and harvested the last ones in mid-January 2014. This was a vast improvement over the prior winter, where we had no cold frame and beginning in mid-December had to literally use a chisel to get the carrots out of the frozen ground! Surprisingly, they tasted fine – until we had a thaw and re-freeze. Then they were mush.

This winter (our fourth), we’re expanding our cold frame efforts. We will have six cold frames – built from “culled” wood and covered with rigid plastic. We hope to harvest spinach, lettuce, carrots, beets, and leeks throughout the winter. Sure, we’ll have to remember to vent them most mornings and cover them most evenings but we’ll also have fresh, home-grown veggies all winter.

Improvements, maintenance, and which veggies to try
Cold frame function improvements can be explained using the analogy of layering clothes for the coldest days in the winter – each layer and the tiny airspace in between keeps you toasty warm. By using floating row covers inside your cold frame or sheets, blankets, or tarps over the cover, you may be able to improve cold frame heat retention. This will only be necessary for periods of extreme cold.

CSU trials of various materials have found the following results:
• Floating row covers provide 2-4 degrees of frost protection and allow sunlight to penetrate so can be left in place permanently.
• Clear plastic sheeting added 3-6 degrees of frost protection.
• When reflective space blankets with the shiny side facing the ground to reflect heat back were placed over plastic sheeting on top of a cold frame, freezing was prevented even when outside temperatures dropped below zero the night following a sunny day. However space blankets (or sheets or blankets) must be removed daily to allow sun to reheat the ground and air inside the cold frame.

Another idea that’s been tested is to string incandescent Christmas tree bulbs inside your frame and plug them in on those super-cold nights. CSU found another 6 -18 degrees of protection was provided by a string of lights and when a space blanket was combined with lights, 18-30 degrees of protection was added. If using lights in your cold frame, as a safety precaution be careful not to have hot incandescent bulbs touch plants, floating row covers, plastic or space blankets.

During extreme cold, use your judgment to determine whether to uncover your cold frame during the day. The critical factors are air temperature and cloud cover because solar re-heating of soil must happen for the box to retain warmth at night. Making these decisions is more of an art than a science and can just be a matter of trial and error. If you can find an inexpensive thermometer to hang inside your cold frame, you can check daytime temperatures yourself. For the curious, a remote wireless thermometer inside a cold frame will enable you to see exactly how cold the cold frame gets during extreme cold.

Another consideration is wind. We get many windy winter days where reported wind speeds can be 60 mph or greater. After investing the effort to build and use a cold frame, you probably don’t want it to blow to Kansas. Some of my plants are taller than my cold frame and their foliage touches the cover. Plant tissue may be damaged if in direct contact with the cover. In the past, I have cut off some foliage of carrots, so that it will not come in contact with the cover, leaving just enough to allow photosynthesis to continue (i.e. 5-6” tall). There was no apparent negative effect of doing this. This year, I will trim back foliage from beets and to a lesser extent, leeks.

Finally, resist the temptation to water your cold frame. Condensation will build up daily and I personally have found it rarely necessary to water. At most, you can include your cold frame in your monthly winter watering. Remember, we want to the veggies to remain dormant; we are not trying to get them to actively grow because they are already mature.

What veggies work best in a cold frame? Forget about tomatoes, peppers, or other warm season vegetables. While a cold frame can enable seed starting earlier than normal in the spring by warming the soil, a cold frame will not support a hothouse garden during winter. Instead, a cold frame is a way to keep mature cool season plants in a dormant state until you are ready to harvest them. Good candidates are: lettuce, spinach, and other greens; beets; parsnips; kale; chard; carrots; radishes; turnips; cabbage; Brussels sprouts; and leeks. Most of these vegetables can survive if temperatures are kept in the mid- to high-20s; some can withstand temperatures in the teens. Some, particularly root crops, often become sweeter as a result of cold temperatures and can actually withstand a freeze and remain edible as long as they are not subjected to freeze-thaw cycles.

Why do I think it’s worth considering a cold frame in Colorado? I’ve read books written by people who live in Maine and Nova Scotia who use cold frames all winter long. Sure, we’re at a higher altitude but they’re at more northern latitudes. If they can do it, why can’t we? At least it’s worth a try. For further details, refer to CSU Garden Notes “Frost Protection and Extending the Season” at http://www.ext.colostate.edu/mg/Gardennotes/722.html.