Saturday, March 28, 2009

In the Zone

Clematis "Jackman's Purple" and
Honeysuckle "Dropmore Scarlet"

The USDA Plant Hardiness Zone for this area of Illinois is listed as Zone 5a. Zone 5a has an annual minimum air temperature of -15 to -20 for a plant to survive. In the US, Zone 1 is the coldest and Zone 11 the most hot. Wind chill (a human skin gauge) is not a factor although blowing wind may harm plants.

Virtually all commercially sold plants have a Hardiness Zone rating. The University of Illinois has extensive data on the hardiness of specific plants that you can trust.

You may check the tags included with most plants. I caution that relying only on the tags or catalog descriptions can be trusting your dollars to what might be an over zealous marketing campaign.

The AHS has developed a Heat Zone map and we are in HZ 5. Today, I’m only talking about the Hardiness Zone because of the extensive cold weather we have experienced this winter.

Experience is often the best (and most expensive) teacher for a gardener to find exactly what survives in your yard. It is often different among various places in your yard.

Many of us may loose some of our Zone 5a plants this year. It did help that we had a pretty good snow cover which acts like an insulating blanket. If ever there was a year when we all should have winter mulched, this would be that winter.

Those of us who live in the country, our yards more exposed, will eventually loose some Zone 5 plants. Some factors that may cause loss:

  • Inaccurate zone labeling by seller.
  • Temperatures below minus 20 degrees for an extended time.
  • Little or no snow cover or mulch.
  • High sustained winds on cold days and nights.
  • Not sufficient moisture OR poor drainage in the soil prior to sustained freezing.
  • Too much applied nitrogen.
  • The plant is not surrounded by other plants, fences, or protection.
  • The plant was grafted.

I’ve lost enough “experiments” that I seldom buy perennials for warmer than a Zone 4. It is hard to give up on Zone 5, because every zone colder means less choices.

There is a host of information available that will help gardeners make the right Zone decisions. Web sites:
U of I Extension Service:
Dave’s Garden plant listings:

I don’t have enough space this week to talk about other winter damage such as breakage by heavy snow & ice, sunscald, ring shake, root death, cambial death, winter drying, rodent & deer, salt, frost cracks, and the “Oh MY GOSH Honey, I just backed over your prized pine!”

We’ll just focus on hardiness Zones while reading those beautifully illustrated garden catalogs. We’ve got time enough for cryin’ when the spring winds come.

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