Now that the weather is cooling down, what is a good way to extend my vegetable growing season? NB

Many of us have been enjoying fresh vegetables from our gardens this summer. It’s hard to give that up, but we know freezing temps are a couple of months away. Fortunately, there are some things we can do to extend our growing season.

One of the things we can do is build a greenhouse but most of us are probably not going to be able to do that. If you can, greenhouses are great; especially if you have a fancy one with a heater. But again, for most of us, greenhouses probably won’t be in the budget this year.

However, many of us could build what is called a cold frame pretty economically from either scrap wood or wood from the lumber yard. A cold frame is essentially a mini greenhouse but without an external heat source. Cold frames utilize solar energy to keep the air and soil temperature higher inside than the outside air thereby allowing you to keep growing food up until it gets really cold.

Cold frames will keep the temperature 5 - 10 degrees warmer inside the cold frame. This means that when our nighttime temperatures are dipping down to a few degrees below freezing, you can still have fresh vegetables safe and secure within your cold frame. Even if it gets down into the low 20’s at night, you can cover your cold frame with straw or a blanket each evening to extend your growing season a little longer. Just be sure to remove these covers during the day because they will prevent the interior of the cold frame from absorbing that solar energy and warming up during the day.

Cold frames are great for growing greens such as lettuce or spinach since these fade pretty quickly when it gets cold. With a cold frame you might get an extra few weeks or even a couple more months of fresh salad material depending on the weather.

Cold frames come in a lot of different varieties. Let’s talk about one that is pretty easy to build and maintain. Cold frames should run east and west with the top lifting up via a hinge to provide for a southern exposure. This southern exposure allows it to absorb the most heat during the day.

Cold frames are essentially a rectangular box with a transparent roof that is hinged to provide open air for cooling and circulation. They can be built out of a variety of materials. Concrete blocks make great insulators, but wood will probably be more cost effective. Redwood or cedar are great long-lasting woods, but you can also use untreated wood. If you do use untreated wood, you might consider painting it with a white latex-based paint for added reflectiveness. Do not use wood that has been treated with creosote or pentachlorophenol as these will cause fumes to accumulate inside your cold frame that could damage your plants.

It’s also best to have the back (the north side) be a little bit higher than the southside. This will help water drain off the cover and provide maximum sunlight for your plants.

If you have an old window with the glass intact, these make great tops for cold frames. If not, you can cover your lid with clear plastic. Since plastic can lose heat quickly, you might want to use two layers of plastic just to provide a little extra protection. The height of your cold frame should be sufficient to rise above whatever crop you intend to grow in your cold frame. You could also put a little weather stripping where the lid meets the frame to help keep the heat in. Wind breaks are also a great idea to help protect your cold frame from brisk north winds. This could be as simple as a couple of straw bales but be careful not to shade the bed with your windbreak.

When using a cold frame, you become the thermostat. If it warms up sufficiently during the day, you may need to open the lid to let some cooler air circulate around the plants. On colder days you may want to leave it closed. Either way, unless we have a heat wave, you will want to close the lid at night to help retain the heat.

Water will be key in your cold frame, and you should water in the morning just like you do your regular garden. We water in the morning so that the water can evaporate off the leaves before nightfall. This helps to minimize disease in your garden.

Cold frames are not only great to help you extend your growing season longer in the fall, but they can also help you start your garden earlier in the spring. With a cold frame, because the soil and air within the cold frame are warmer, you can start your seeds or transplants outside before our average last frost day of April 15th. Once we are past the danger of frost, you can either prop open the lid or remove it permanently for the rest of the summer season. Happy gardening!

You can get answers to all your gardening questions by calling the Tulsa Master Gardeners Help Line at 918-746-3701, dropping by our Diagnostic Center at 4116 E. 15th Street, or by emailing us at mg@tulsamastergardeners.org.

Photo by Bernadette Kaufmann from Pixabay