Thursday, August 6, 2015

Map your Frost in August




The biggest frosts follow clear wind-free nights. This coming weekend is expected to give us some beauties.

Frost reaches its peak at that magical time when dawn has achieved full daylight, but no rays of sunlight have yet touched the ground.

That is when you need to be out there in your ugg boots and beanie (and perhaps some other garments in between, if you feel the need), to see just where it lies, and where it doesn't.

People who live in areas well-known for hard frosts are often surprised to find just how many frost-free patches there are. The tedious job of covering tender plants can be unnecessary if they are planted in these patches.

If you carry a handful of stakes and a hammer on your morning walk in the frost, you can mark frost-free spots where little plants will thrive.

Frost flows downhill like a river. It is diverted by obstacles in its way, and it pools in hollows and above barriers. You may find quite large frost-free areas downhill from these barriers.

As well as this, every tree and shrub has a little frost free "aura" around it - a strip, smaller on the uphill side and larger below, but existing on the sides as well.  This strip just outside the tree's dripline (where most of its surface roots exist, and may compete with a new plant), is excellent planting territory. Structures also have frost-free auras, though to a lesser extent.

While planning this year's spring plantings, you might also look for places to create frost shelters for the future. These can be rows of frost-hardy shrubs and trees, perhaps doubling as hedges, screens or windbreaks. They are particularly efficient as frost-breaks if  they are bow-shaped and placed across the direction of frost flow with the centre of the bow uphill from the ends. This diverts the flow outwards, creating a sheltered nook to be filled with tender plants in a few years' time.

Forests and shrublands tend to hold the day’s warmth overnight. In our part of the world, ground temperature in these environments is always above freezing point and frost tender plants grow in them with no trouble at all. Revegetating with the same species can be a problem where clearing has thrown the area open to frost.

With time and careful planning, however, it is possible to eliminate frost and to restore the full range of native plants that once grew there.
  
BUT WHAT ABOUT BLACK FROSTS? 
Those horrible sneaky things! You don't see them, but the plants get frosted all the same.
The frosts we see are made of frozen water. That's why they are white. There is usually at least some moisture in any air, and it turns white when it freezes. If the air is very dry, however, it reaches freezing temperature just the same, with no revealing whiteness to show where it has been. That's a "black" frost. Frost is really all about temperature, not about whiteness.
Black frosts do the same amount of damage, which means that they still freeze the moisture in the leaves (which is what frost damage is), but there is no whitened landscape to show what's happening. Dry air freezes more readily than damp air, so it gets colder. This is the reason  black frosts can do the most damage of all.

Obviously, there is a continuum, between white frosts and the black. The whitest one is really the one with the most air moisture, not necessarily the coldest.
Those white, white frosts are the ones to look for though, when it comes to planning our planting. They can be so helpful, painting a clear picture to show us where the cold air goes, in our own little piece of the landscape.


2 comments:

Ross and Jen said...

This is great information - thanks, has been really helpful to a gardener in just the second winter in this area.
Thanks very much :-)

Patricia Gardner said...

Nice to hear from you, Ross and Jen.
I hope you find yourselves doing some happy and confident planting this spring. Only a few weeks more to go!
cheers,
Trish