My vegetable garden is starting to slow down. I know the end is coming, but how can I extend my growing season so I can enjoy fresh vegetables a little longer? LW

We can kind of feel it in the air, can’t we? At this point we are just a little over a month away from our average first freeze date of Nov. 15^th. That first freeze is an event that typically shuts most of the garden down. However, there are some things we can do to not only extend our growing season a little longer this fall but then use to start a little earlier next spring.

First of all, a greenhouse is a great option but many of us don’t have space or the budget to allow for the building of a greenhouse on our property. If you do have space and the budget, greenhouses are great, especially if you can install a heater. With one of those, you should be able to grow veggies almost all winter.

For the rest of us, something called a cold frame is an economical option. Cold frames are usually small enough that they can be built from scrap wood or wood from the lumber yard. Cold frames are small greenhouses, typically no more than a foot tall, 3 feet deep and 4 feet wide. But they can be any size you would like.

They are called cold frames because they don’t have an external heat source installed. Instead, they use solar energy from the sun to keep the air and soil temperature at a level that is higher than the outside air. By utilizing a cold frame, you can extend your growing season in the fall up till the point we start getting really hard freezes.

Inside the cold frame, temperatures can be 5 to 10 degrees warmer than the outside air. This means that as our nighttime temperatures get down to even 22 degrees, you’ll still be able to keep your veggies safe in the cold frame. When it gets colder than 22 degrees at night, if you are really committed, you can cover your cold frame with a blanket each evening for a little extra insulation. Just be sure to remove the blanket during the day so the sun can warm your cold frame back up again.

Cold frames work great for growing lettuce or spinach since these give it up pretty quickly when it gets cold. By using a cold frame for your greens, you might get a few more weeks of fresh produce or even longer, depending on the weather.

There are a lot of different ways to build your cold frame but typically they are rectangular boxes with a transparent roof that is hinged so you can open it during warmer days to increase air circulation. Plan on placing your cold frame so that is runs east and west and a top that opens to provide southern exposure. Southern exposure increases its ability to absorb heat from the sun.

While concrete block construction will provide a bit of insulation, most cold frames are constructed out of wood. Redwood and cedar will last the longest, but untreated wood will also work. If you do use untreated wood, think about painting the structure to help it last a little longer. Do not use wood that has been treated with creosote or pentachlorophenol since fumes from them will accumulate inside the cold frame and possibly damage your plants.

Also plan on having the back side of the cold frame be higher than the front to encouraged drainage and allow for maximum sun exposure.

Old window frames that still contain the glass make great tops for cold frames. If you can’t find one of those at a garage sale or estate sale, you can stretch clear plastic over the cold frame top. Plastic isn’t a great insulator so maybe plan on a couple of layers of plastic to help retain heat. A little weather stripping between the frame and the lid is also a good idea.

As far as height of your cold frame goes, it should be tall enough to rise higher than whatever crop you plan on growing inside. A wind break of some kind isn’t a bad idea either if your location will be exposed to the cold north winds. Wind breaks don’t have to be fancy and could consist of something as simple as a couple of straw bales.

Essentially, you will be the thermostat for your cold frame. If it gets warm during the day, you will probably need to open the lid to let some cooler air inside. On colder days you will probably want to leave it closed. In most cases, you’ll want to leave the lid closed at night to help retain the heat.

Plan on watering the plants in your cold frame just like you do during the standard growing season. Remember, watering in the morning is best so the water can evaporate off the leaves during the day. Doing this helps to minimize disease.

I mentioned that cold frames can also help in the spring. Because the soil and air are warmer inside your cold frame, this allows you to start your veggies outdoors a little earlier. Typically, our last average frost date is April 15^th, but with a cold frame you might be able to get started a few weeks earlier. Then, once the danger of frost has passed, you can either leave the lid propped open or pack the cold frame away until fall. Either way, cold frames are a great way to extend your growing season. See you in the garden!

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: KVDP, Public domain, via Wikimedia Commons