Extending the Season

published on 13 September 2022

How to keep your garden growing as the weather cools

Guest Contributor: Cassey Anderson, CSU Horticulture Agent

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Extend the season with a simple cold frame
Extend the season with a simple cold frame

Perhaps by now you are growing tired of your garden; the weeds, the endless zucchini, (or, in my case this year, cucumbers) and you’re ready for a frost to come and take it all away. However, if your goal is to contribute produce for as long as possible to your favorite local hunger relief organizations (hint! hint!) then season extension could be a great option to investigate.

There are varying levels of season extension depending on your commitment and your goals. They range from a sheet of plastic over your plants to a full greenhouse. We’ll go through what you’ll need for some basic season extension options. 

Row Cover 

Interested in learning more about Floating Row Covers? Check out this video from our partners at University of Colorado's Grow & Give program!

The material typically lays directly over the crop and is made with woven or spun fabric. The use of row cover can increase temperatures by up to 15 degrees F during the day, but only offers 2-4 degrees of frost protection in spring, slightly more in fall with warmer soil temperatures. It’s important to keep the cover secured against the ground with rocks, sand bags, soil etc. especially if you’re in a windy area or if you suspect a particularly cool evening coming up. 

Plastic mulch

Pepper transplant in plastic mulch, Rebekah D. Wallace, University of Georgia, Bugwood.org
Pepper transplant in plastic mulch, Rebekah D. Wallace, University of Georgia, Bugwood.org

This is a great way to warm the soil a little especially in the springtime. Plastic mulch does little to help with freezing air temperatures, so it is limited in its use for extending the season for warm season crops until air temperatures have warmed. Plastic mulch can give a jump start to germination for warm season crops at planting by warming the soil and speeding germination. Different colors of plastic mulch have different benefits. Clear plastic will warm the soil the most, but weeds are likely to grow in. Black plastic will warm the soil more slowly but is better at weed reduction. For soil cooling, white or reflective mulches are an option and research has shown they can reduce some insect pressure. 

Low Tunnel with plastic 

Low tunnels install quickly and work well in an established garden
Low tunnels install quickly and work well in an established garden

To build a low tunnel you need some type of support and UV resistant plastic. Generally low tunnels can be built over a standard garden bed whether it is a raised bed or in-ground. These are simple to install using rebar as ground support and PVC pipes bent over the garden bed. Fancier versions are out there if you look around. You can put spun row cover over your support structure but will get substantially better cold and frost protection with greenhouse plastic. Spun row cover is better at providing insect or sun protection. Avoid purchasing inexpensive sheet plastic like for painting projects unless you want to be cleaning up shreds of tiny plastic for the rest of your gardening career. Instead seek out plastic designed for greenhouses. On warm days it is good to have a way to vent the system even if that just means unclipping the plastic cover and pulling it up.

Cold Frame

A cold frame can be an easy DIY for the home garden
A cold frame can be an easy DIY for the home garden

A cold frame can be easily built DIY with wood and an old window. More permanent structures can be made with cinder blocks or bricks. Ideally, face the cold frame south or west to take advantage of the warmest part of the day. You can purchase temperature sensitive hinges that will open as the cold frame heats up, reducing the chances of cooking your plants if you forget to open them on a warm autumn day. These are effectively miniature greenhouses. Sometimes they are portable so you can move them around the garden if you so desire. 

High tunnel 

High tunnels for larger scale growing
High tunnels for larger scale growing

High tunnels are more involved than most homeowners want to go, they are a semi-permanent structure that give you a lot of growing area. Most are made using a UV resistant polyethylene, in one or two layers built over a metal frame. There are numerous kits available online. When planning it is important to have a way to vent the plastic during the day. 

Greenhouse  

The bigger the better for improved thermal mass which leads to less temperature fluctuation. Greenhouses are the most permanent season extension option, and with heat can be used to grow year-round. They are also the largest investment in both time and money. Contact your local extension office if you want to figure out an ideal greenhouse for your space. 

Add some holiday cheer

Incandescent lights are a quick - and pretty! - way to keep crops warm
Incandescent lights are a quick - and pretty! - way to keep crops warm

If you’re not looking to build a heated greenhouse but you need to get your crops through that first one or two light freezes to get a few more harvests, you can supplement the heat by using incandescent “C4” Christmas light strands. The heat they put off will provide you with many degrees of frost protection. It does require access to power. You could also use something like a thermal cloth or numerous layers of plastic to achieve greater frost protection. A good seal against the ground is very important for successful frost protection, as the heat in the ground will radiate up, and can be lost if there are gaps. 

If you have questions about any of these options for season extension, or any other yard and garden questions be sure to reach out to your local Extension office for more information. 

Next month we’ll be looking at popular things to do in the garden and whether you should too (crop rotation, cover crops etc.)! 

Enjoy your autumn harvest!  As always, contact your local county Extension office if you have further questions. 

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