JUMP START ON SPRING
Now that the holidays are over, it is time to start thinking Spring!!
Cold frames (or hotbeds) are simple structures that have two main purposes. They act like miniature greenhouses to trap radiant heat and to provide protection and insulation from the elements. They can be very elaborate, or very simple, depending on the expertise of the builder, and what you have on hand, or are willing to pay for materials. You can even purchase a pre-made one.
Even though they can be simple or elaborate, there are a few certain basics that need to be observed. First thing to consider is the location. Ideally it should be South facing to collect the warmth of the sun. A West facing direction would be second best. When I say facing a certain direction, the sloped front should face South or West. The sloping will of course give you better sun exposure, but it is not so critical that all is lost if you don't have it.
Most cold frames are a simple rectangular wooden box, about 2-3 ft high that sits on the soil surface. If you have access to straw bales, maybe something left over from the Fall holidays, you can use them too. That would look something like this.
Other materials to consider are brick, masonry, cinder, and concrete blocks. Use your imagination. If you really want to get fancy, foamboard insulation panels can be used inside the frame on the above-ground, North-facing side for even more insulation.
Good drainage is essential for the plants, especially if you are planting directly in the ground and plan on moving the cold frame later. If you are using this to harden off or start seedlings in containers, just make sure that the containers can drain. An accessible water supply is also very important. The top covering will prevent rain from watering your plants, so that must be done manually.
Okay, you have decided what you want to build the walls out of. Before you begin building, you need to decide what the top will be made out of. Follow me for a second here. Many different materials, both recycled and new, can be used for the lid of the frame. These include such options as glass, fiberglass, or poly (plastic) film.
Double glazed windows are a good choice. They are durable and are heavier than other materials. Glass is generally looked upon as the best material to cover a cold frame. Other materials include discarded storm windows from screen doors or no longer needed patio doors. If polyethylene plastic is used, the film should be clear and at least 6 mil thick. Consider using a double layer for extra insulation. The poly is not very durable and will probably have to be replaced each year.
Now, why did I say that you need to decide on the top covering before building the box? If you have a shower door that is 7 feet long and you build a box 8 feet long, how effective do you think the whole thing will be? Get the measurements of your top first, then build your box.
There are some pros and cons to the different materials used for the top.
If you use glass:
*Recycled windows can be used.
*Good light transmission.
*Good insulation value.
*More hail-proof (weatherproof) than polyethylene.
*Glass is heavy. The extra weight means the frame must be able to support it. It also makes opening and closing the lid more difficult.
*Broken glass is more difficult to replace and repair.
*Expensive to purchase new.
If you use plastic:
*Inexpensive to purchase.
*Easy to install and lightweight to handle.
*Probably will need to be replaced each year
*Won't withstand large hail stones, heavy snow, ice loads or errant twigs.
*Must be secured so that it doesn't take off in a strong wind.
There are other materials that can be used, again each one will have its pros and cons.
I am by no means a carpenter, that is my brother's gig. That sucker could put McGyver to shame. So, I am not going to give you building tips or dimensions or anything like that. Besides, I have no idea what size shower door or glass windows you are going to use. I will give you some ideas on how to use your new built toy.
You can start the Spring cool weather vegetables - Lettuce, Onions, Spinach, Radishes, etc. - up to 45 days before you experience your last frost. While excellent for starting your Spring vegetables, a cold frame is NOT the best place to start warm-weather vegetables such as Tomato or Pepper plants. The average temperature may not be consistently warm enough to germinate those seeds. What you can do is, long before Spring arrives, start the plants indoors under grow lights, and then move them out to the cold frame to help harden them off. This eliminates a lot of the transplant shock.
You can also use a deep cold frame in the Fall if you wish to extend the growing season of the same vegetables.
There is one more very important factor that I should tell you about. Even in the middle of Winter, there will be very sunny days. The temperature in your box, under glass, could get very hot. If you are doing the cool season crops, Lettuce, Cabbage and such, they will not be happy. Whatever type of covering you decide on needs to be hinged in some fashion. This way when the temperature gets too hot, you can prop it open, like this.
Remember to take into account wind speed and direction. You don't want the lid flying off or the interior temperature to cool down too fast. This prop job is extreme, sometimes only a couple of inches will suffice.
I wish all of you the best of luck this growing season. May your lettuce produce large leaves and your zucchini produce an abundant crop. Beware that last wish, I have heard evil stories of such bountiful crops of zucchini that people were locking their car doors at church, in fear that a fellow gardener would leave a present of their "OVER BOUNTIFUL" zucchini on the driver's seat!