It's late February as I write this and, after a couple of weeks of wonderfully warm weather, we have dropped back to freezing temperatures. Personally I don't mind the cold (although I am ready spring by now) and many of my trees don't mind the cold either. In fact, they need it.

Winter, and the cold weather it brings, is a necessary and vital part of lifecycle of many trees and shrubs. It brings a period of rest, akin to sleep for animals, that we call dormancy. Without this dormancy the tree will eventually "burnout" and die.

Dormancy is typically most evident in deciduous trees. First, as the flow of sap slows down, many tree's leaves start to turn shades of yellow, red and orange. Then, as their need diminishes, they fall off their branches leaving them bare for the winter.

A lesser dormancy occurs for many trees at the height of summer as daytime temperatures and available light reaches it's apex, but that's another article.

My Coldframe

All this is important to keep in mind as the seasons start to change and things start shutting down for the winter. However, there are many trees in my collection that would not make it through the coldest parts of our rather mild winters. Those are the tropical and sub tropical plants, i.e. ficus, buttonwoods, and bougainvillea. For them I have built a small greenhouse using 1x2 timbers and plastic sheeting. I then used a small space heater for those nights when the temperatures dropped dangerously low.

A friend had mentioned in conversation the concept of using temperature ballast in greenhouses. It consists of keeping large containers of water in with the plants. The water absorbs the heat when the heater is active and then slowly releases it when it's not. Daytime light and heat also helps to keep the water temperature up. This provides a more stable ambient temperature for the plants. It also helps keep the humidity up. It's worked very well thus far.

As I had stated previously, we had a number of warm weeks just before the second cold front hit us. A couple of my trees that I would normally have outside in the cold started to come out of dormancy and leaf out. This is a problem.

Although the trees have been in the cold all winter and can survive it quite happily, once they start to leaf, they become susceptible to the cold. A sudden cold snap could kill it's new leaves and buds, forcing it into developing new buds when it begins to warm up again.

For trees in nature it's no big deal. After all, with roots that stretch for many feet in every direction, they can afford the extra drain on it's resources. But a bonsai, with it's limited roots/resources, news leaves can make the difference between keeping or losing a branch or, in the worst of situations, it can mean life or death for the tree.

My Crape in just two weeks.

That's why I brought those "early risers" into the greenhouse with the tropicals. A little protection is all they need to assure a healthy and vigorous growing season.

Spring is almost here and I hope it's a good one for you and your bonsai.