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.