Cold Frames, Tunnels and row covers

 Getting an Early Start – Cold frames, Tunnels and Row Covers

 If you are a gardener that is anxious to get growing in the spring and don’t have a greenhouse, you may want to take advantage of several season extenders available to gardeners. Cold frames, row covers, tunnels and individual plant protectors are widely available and easy to use. These items can give you an early gardening start and let you protect crops at the end of the season too.

Do remember that patience is needed and there is such a thing as starting too early, even with protection. Cold frames, tunnels, row covers and other protections generally aren’t used for more than 6 weeks before plants could be planted outside. They take the place of starting seeds indoors or are used to harden off seedlings started inside. Read your seed packet label to know when to start seeds and learn what planting zone you are in before beginning.

Cold frames

Cold frames are a box with a clear top and sometimes clear sides. Snug in their protected world plants get the advantage of natural light without drafts and frost. Cold frames are used to start seedlings or to harden off, (acclimate), plants that were started inside. In the fall they can also be used to grow a crop of greens before severe weather sets in.

A gardener can simply construct a box with a glass, Plexiglas, or a heavy, clear plastic film top. Cinder blocks or old lumber  can form a cold frame. Old glass windows are excellent tops. Clear vinyl shower curtains are tough, easy to find and relatively cheap and can be stapled to a frame as your top. The sides of the box can be 2-inch-thick Styrofoam panels cut into pieces and taped together, which makes an inexpensive, light, easy to move box.

The box should be at least eighteen inches deep at the low side to allow plants to grow. The walls should be thick or well insulated. You can bank soil around the outside of the box or use bales of straw around it. (You can even use straw bales as the walls for your box). White walls or walls lined with reflective material help maximize light.

You can add a floor or simply have the box sit on the ground. If it sits on the ground add a layer of thick cardboard, Styrofoam, straw or other insulating material to the bottom. A layer of black plastic over this floor will help absorb heat to keep the box warmer overnight. You can purchase heat cables or mats from garden supply stores for bottom heat if you like.  Don’t use regular heating pads.

The cold frame is best placed on the south side of a building in full sun. The back should be higher than the front so the top slopes down. This allows more sun to enter. Ideally the box should face south although west or southwest can work.

Purchased cold frames may be made of wood or plastic. They often have hinged lids that are connected to a device that opens the lid when a certain internal temperature is reached. They may also have heat cables and fans to circulate air. 

You can buy the devices that will raise the lid of a homemade cold frame. You can also add a small fan if you want to circulate the air. Point the air direction over the tops of the plants.  Solar powered fans exist, they are sometimes sold in auto departments to ventilate cars.

Set the cold frame up several days before you sow seeds or set plants in it so it will warm up. Some people add soil and plant directly in the cold frame, but plants transplant better if started in pots or flats. You may want to germinate seeds indoors, in warmer nighttime temps, and move the pots or flats to the cold frame once the seedlings have their first true leaves. 

Using the cold frame

Don’t start seeds or plants in a cold frame too early. When they outgrow the coldframe the weather should be ready for them to be planted in the garden.  Planting in a cold frame can usually begin six weeks before your last expected frost.

The most important thing to remember about coldframes is that even though it is in the upper thirties outside on a sunny day, it will be much warmer inside the box with the lid closed. If temperatures get too hot the plants will die just as quickly as if they got too cold. Buy an inexpensive thermometer to put in the coldframe. When temps reach 80 degrees the top should be raised to allow ventilation. This can happen in happen in an hour or so of late spring sun shining on the box.

If you don’t use a thermostatic lid opening device, you must be diligent in raising and lowering the lid depending on weather conditions. If you work during the day this factor can be tricky and you will need to become a close observer of weather forecasts. If the day is to be sunny and above 40 degrees, the top should be propped open at least a few inches. On cloudy, cool days the box top can be left down.  If the day is sunny and going to be above 60 degrees, you may want to prop the lid fully open or remove the top.

Close the box lid about an hour before sundown to help preserve heat. If extremely cold weather threatens the whole cold frame can be covered with a blanket. Normally plants will be fine if night temps fall to around freezing outside the box or if the box is covered with an additional layer even down to the mid-20’s. 

 

Tunnels and row covers

 

Tunnels are tall enough to walk under; row covers are shorter and must be removed before caring for plants. Home gardeners are more likely to use row covers. They are generally covered with plastic or a spun polyester-like fabric. Tunnels always have a frame of some sort, but row covers may have a frame or just be loosely laid over plants. 

Usually tunnels and row covers are rounded, hoop like structures although some may look like inverted V’s. Fence panels can be bowed to make a tunnel or PVC or metal pipe can be bowed to use either as a tunnel or row cover. They are then covered with plastic or spun fabric.

There are some lightweight spun fabrics that will not harm plants if it touches them and lets air and water pass through. It can also be laid loosely on the plants and will lift as they grow. Plastic needs a frame as it should not touch plants and must be removed before watering. 

Using tunnels and row covers

Both tunnels and row covers are used over crops planted right in the ground or plants in larger pots. They can protect crops from frost and raise the temperature in the daytime to promote growth. 

Row covers will give you about a month’s head start over the same plants planted without them but won’t protect plants in a hard freeze. Wait until the chance of hard freezes- (below 30 degrees)- is over before setting up sensitive crops like tomatoes and peppers under them. Plants like lettuce, peas, radish, cabbage, kale and a few others will be fine through temps in the 20’s.

Another consideration of tunnels and row covers is snow. If you get a late season snowfall that is heavy enough to flatten the tunnels or row covers your plants can be damaged.  Remove the snow as soon as you can and hope for the best. Inverted V shaped tunnels may work better than rounded ones in snow country.

In cooler areas, crops like melons and peppers benefit from draft protection and the warmer daytime temperatures a cover provides early in the season.  Row covers are great to protect crops at the end of the season when an early frost threatens also.

 

Care must be taken to lift the row covers on a sunny day, especially plastic ones. Ventilated plastic may be fine when temperatures are below 50 degrees but keep a close check on plant conditions. Thin spun fabric covers let some air through but even those can become too hot.  When temperatures regularly rise above 70 degrees all covers need to be removed for the season.

Individual plant protection- (cloche)

There are plant protectors that fit over individual plants. The formal name for these is a cloche. Some are filled with water to hold heat at night. You can buy these or devise your own. Small plants can be protected by 2-liter drink bottles or gallon milk containers with the bottom cut off.  Set it over plants and leave the bottle caps off for ventilation. 

Remember that if you use plastic or glass covers the protection will need to be removed on warm sunny days. If you forget your plants may be cooked. These kinds of covers are generally used for 2-3 weeks before the last frost. You may only need them if temps are forecast to go below 36 degrees at night.

Small portable greenhouses

These have become extremely popular, and they do have uses but also some limitations. They are usually a 3-4 shelf unit about 3 feet wide with a clear vinyl cover. You can use them inside to increase humidity and warmth for seedlings or outside in a protected area. If you use them outside, try to place them against the south side of a building. You must be able to anchor them so they won’t blow over in the wind. Either wire them to an existing wall, post or rail or put posts in the ground to tie them down. They need to be anchored near the top as well as the bottom.

Be careful of the amount of weight you place on the shelves of these units.  Some of the least expensive ones don’t have very sturdy shelves. And just as with other types of season extenders they need to be opened in sunny warm weather. Usually these have a zippered opening on one side. If it gets above 70 degrees this opening may not be sufficient to ventilate the unit enough and you may need to remove the whole cover.

Milk jug hack

You’ll notice I didn’t mention planting seeds in milk jugs, often called winter sowing. That’s because these are suitable only for seeds that need cold stratification – (winter cold and freezing and thawing) and when you only need to start a few plants with those seeds.

A small milk jug is subject to wide swings in temperature in spring. Just leaving the cap off isn’t sufficient ventilation in many cases. Drainage is usually poor too. One to four seedlings per jug would be ideal but people crowd them with seedlings, that become stressed and stunted. It’s a cute hack but not what good gardeners want to use.