edible notables
FREEZE FRAME
Using planting boxes to help extend the growing season.
WRITTEN AND PHOTOGRAPHED BY SUSAN DONALDSON
When was your first freeze this fall? In Northern Nevada, it’s not unusual for us to have a frost any time after August, although sometimes our first freeze doesn’t occur until November. That kind of variability makes gardening interesting here, but it also makes a strong case for using season-extension techniques to deal with early-fall and late spring frosts and freezes.
One way to cope is by using a cold frame. Very simply, a cold frame is a planting box with a clear lid, which is heated by the sun. The temperatures inside the box can get to about 5 degrees to 10 degrees warmer than the outside air — just enough to lengthen your growing season by a month or two.
One use of cold frames is to plant crops such as lettuce, beets, or radishes to provide fresh vegetables for the winter. Last year I tried a salad lettuce called mache that continued growing through the winter.
Most cold frames are built directly over the soil. Ours is on a south-facing paver patio to collect more heat and provide some shelter from the wind. We built the box deeper than normal to provide room for our planting medium. Since it’s above ground, we have more heat loss than we would if planting directly in the ground. We also added blankets over the top for more protection on cold nights.
The cold frame is a rectangular box. The back is higher than the front, making for a sloping lid. We used the glass from an old patio door for our lid, but plastic with good light transmission also works. The lid should slope at least an inch per foot. Hinge the lid so you can prop it open for ventilation.
The frame usually is made from wood, but you can use concrete blocks or bricks. Avoid materials treated with wood preservatives that are toxic to plants. Redwood or cedar lasts longer. Don’t make the frame wider than you can reach across.
Place a thermometer in the box to monitor the temperature. Cool-season plants prefer an air temperature of 65 to 70 degrees during the day and 55 to 60 degrees at night. On sunny winter days, unless you open the lid, it will probably get too hot for many crops.
To harden transplants, open the lid wider and longer each day and overnight when temperatures allow. If you have one of those late-spring frosts that we’re famous for, you’ll be all set to protect them with your cold frame!
For questions, call the University of Nevada Cooperative Extension at 775-784-7070 or visit http://www.unce.unr.edu.
Susan Donaldson is water quality and weed specialist for the western area of the University of Nevada Cooperative Extension.
