Baby, It's Cold Outside

For the past couple of weeks,

Minnesota, along with much of the country,

has been in the deep freeze.

What's happening to our perennials out there?

There are a few ways that perennial plants can be damaged

during this extremely cold weather.

The first way is by root-killing temperatures.

Plants are categorized into hardiness zones, meaning

the temperature when the roots are killed by the cold.

Zone 3 plants should survive temperatures to -30 below.

That's where it gets tricky. 

Snow is a great insulator, and can elevate the temperature

underneath it, protecting the plant in exceptionally cold weather.

According to researchers at Rutger's University, a 9" snow cover

can make a 42 degree difference in temperature.

So, you are probably OK if there's snow cover on your perennials.
If there isn't, you may discover some losses next spring.

If there isn't 8" or more of snow on your perennial bed,

then get out there and shovel some on to keep them cozy.

If there isn't snow on the ground, row cover can help.

Using polypropylene fabric instead of burlap is easier on the plants,

because the burlap is exceptionally heavy when wet.

Some excellent information on plant hardiness from the U of M Extension:

Roots do not become dormant in the winter as quickly as stems, branches and buds,
and roots are less hardy than stems. Roots of most trees and shrubs that grow in Minnesota
are killed at temperatures at or below 0 to +10°F.  These plants survive in Minnesota because soil temperatures normally are much higher than air temperatures and because soil cools down
much more slowly than air temperature.

Many factors influence soil temperature. Moist soil holds more heat than dry soil, so frost penetration will be deeper and soil temperatures colder for sandy or dry (drought) soils.

Snow cover and mulch act as insulators and keep soil temperatures higher. With newly planted trees, cracks in the planting hole backfill will allow cold air to penetrate into the root zone,
reducing fall root growth or killing newly formed roots.

To encourage fall root growth and to reduce root injury, mulch new trees and shrubs with 6 to 8 inches of wood chips or straw. If the fall has been dry, water heavily before the ground freezes to reduce frost penetration. Check new plantings for cracks in the soil and fill them with soil.

The idea is to be sure there is some protection in the winter.

An ounce of prevention is worth a pound of cure.

The next problem is winter desiccation. Humidity levels are low 

in winter, and the wind can dry out shrubs and conifers.

Desiccation happens because the ground is frozen and the tree is losing moisture

faster than it can make it up using it's reserves.

Keeping evergreens properly watered throughout the growing season and into the fall is a great
way to reduce winter injury. Never stress plants by under- or over-watering.
 Decrease watering slightly in September to encourage hardening off, then water thoroughly in October until freeze-up. Watering only in late fall does not help reduce injury.  

Mulch is another great way to conserve moisture for the tree in winter.

If your evergreen has winter damage:

wait until mid-spring before pruning out injured foliage. Brown foliage is most likely dead and will not green up, but the buds, which are more cold hardy than foliage, will often grow and fill in areas where brown foliage was removed. If the buds have not survived, prune dead branches back to living tissue. Fertilize injured plants in early spring and water them well throughout the season. Provide appropriate protection the following winter.

Other factors in winter that can affect plant longevity and survival are

rodent damage, especially bark removal by rabbits or deer

ground heaves, where a repeated cycle of cold, then warm lifts the plant's roots out of the soil

and exposes them to cold temperatures

applying excess nitrogen in the fall to promote late, leafy growth

which will weaken the plant going into winter

The best way to ensure winter survival is to take some precautions in the fall, purchase 

plants that are reliably hardy in your zone, and to make sure your plants are healthy 

going into their winter dormancy.