Wednesday, July 27, 2011

Snow business

Gardens covered in snow are everyone’s fairytale dream. We see movies about Christmas with snow falling gently on the garden and trees. It’s romantic. It’s fun. It’s cosy inside.

Out in the garden though snow can cause a few problems.

Let’s firstly dissolve one myth. Snow is cold and the cold snow will harm plants. This is not true. Snow by itself is at freezing point or just a little below that and it does not get any colder. In the mountains snow acts as a sort of thermal layer between the really cold stuff frost, ice and icy wind. The alpine plants love it under snow. They are protected from the real cold and survive very nicely. In spring when the snow melts they get watered as well.

Cold is a bad thing for some plants but the really harmful cold is frost and ice and icy wind. Tender plants from warmer climates do not have outer layers for protection from the cold and ice. The damage to these plants is caused by water inside the individual plant cells freezing. When water freezes it expands. When water inside a plant cell freezes and expands the cell tissue is destroyed. Have a look at tender plants after a frost and notice that the leaves will turn black very quickly that day and the leaves, or stems or both will die.

Protection of plants from frost damage is usually provided by covering the plant/s with a layer that will stop the frost from getting to the leaves. There are products on the market that can provide this sort of protection but in early years sacks and other material was used very successfully.

So what damage does snow cause?

The biggest and most notable amount of damage caused to plants by snow is breakage. It is the weight of snow sitting on a branch or branchlets that cause them to break. Branches are only used to carrying leaves not the weight of snow.

Throughout Canterbury this winter many, many trees have been damaged by branches laden with snow and then breaking under the strain.

To prevent the loss of branches the best thing to do is to remove the snow. This can be done by shaking smaller trees and shrubs to get the snow to fall off.

Another method for larger plants would be to hose them down with cold water from the hose. The water may freeze but that will not cause a great deal of harm to the plant. For many years the apricot growers in Central Otago used a water spray to cover their orchards when frost was forming. This was because when water freezes it actually gives off latent heat and the temperature does not drop much below freezing point. However a frost can take temperatures well down below freezing point and destroy cell structures.

Broken branches are a major snow damage problem. There is little that can be done when the branch is broken. Preventative action like removing snow is important.

Broken branches are a problem. Once a branch is broken there is little that can be done to repair it. Therefore careful pruning is required to remove the branch. Sometime complete removal is necessary other time only the broken piece needs to be removed. When removing cut back to a branch join to avoid stubs.

If branches a r split along the branch removal of the weight of snow is critical. Split branches may repair themselves with a little permanent help. Take some of the branch weight off by trimming the branchlets back. Attach a wire from the split branch to a solid branch and tighten it up until the split is closed. Leave the wire in place in the tree for the rest of its life as the branch will be a lot weaker.

Falling and settling snow is a lovely thing but can cause damage to plants. To prevent major damage  to important trees and shrubs remove the snow by shaking the  plant or using cold water from the hose.