Tip of the Week  #18

As mentioned in the previous article, an unheated garage or tool shed are good places to winter your bonsai collection, but the best way to winter them is by making a cold frame.  If you have a patch of dirt available, you qualify.   The first thing you need to do is to determine the "frost line" in your area.   The frost line is the deepest point to which the ground will freeze in a given area. A plumber should have this information because water pipes must be laid below this point to assure that they do not freeze and burst in winter.

Your local department of agriculture is another place to look, or check with a bonsai club or society in your area. Your cold frame pit should be dug in a shaded location to prevent overheating from sunlight. Dig the hole at least four to eight inches deeper than the frost line. The reason is that the earth below the frost line, by definition, is always above freezing temperature. This means that during periods of severe cold, the ground will actually heat the air, and because heat rises, the entire inside of the cold frame will be warmed. Depending on the outside temperature, construction of the cold frame, sun, wind, and other factors, your bonsai may freeze, but it won't be nearly as solid, or long lasting a freeze as at ground level. The smaller the cold frame, the better it will work, so don't just dig a huge pit.

To determine the best size, place all your bonsai on the ground, close to each other, but not touching. This will allow for future growth. Measure an area a couple of inches larger than the bonsai occupy, and you have the dimensions. If you are planning to add to your collection, you may want to make it a little larger. Now dig a hole the size needed, and four to eight inches deeper than the frost line. While not always necessary, depending on the consistency of the soil, it's a good idea to line the inside of the hole to prevent cave-ins. Wood can be used, but will rot in time. Fiberglas sheets, available at lumber yards and home improvement centers, are a good, long lasting, economical alternative to wood. If you cut some pieces of PVC pipe about 6 inches longer than the depth of the hole, and drive them into the ground against the fiberglas walls, you should be set. For the most effective cold frame, build an enclosure with a lid around the pit above ground.

For some ideas on how to build the enclosure, or for some high quality, affordable alternative to "Do It Yourself" look at the descriptions and photos of the Cold Frames, we offer for sale.

The lid should be either removable, or be able to open completely. When it rains, open or remove the cover to get your trees watered. If it snows, do the same. If the snow is deep enough, shovel it into the pit when the snowfall is over, then close the cover. I know that sounds strange, but snow is a great insulation, and it will melt slowly, watering your trees.

A good cold frame is not the easiest thing in the world to make, but you only have to do it once, and after your first year of wintering in a cold frame, you'll bless every minute you spent on the project. If your collection is worth the effort, by all means, make the effort.