Sizzling temps: Misting garden plants can reduce heat damage

August 31, 2011 02:51 pm

Previous columns have mentioned the American Horticultural Society (AHS) Heat Zones and what the knowledge of a particular plant’s zone could mean to you. Prior to the past two weeks, writing about Heat Zones seemed somewhat unimportant for our region this growing season, but obviously that has changed with the past 10 days of warming weather.  

Heat Zones are becoming more widely used on plant labels. Many of my local purchases this year had both the USDA hardiness zone and the AHS heat zone clearly defined on the label. The importance of adding a heat zone to a plant description serves several purposes including, if for no other reason, increasing our knowledge of where we should place the plant in the ground once we get it home.

Heat zones were originally developed to highlight the effects that prolonged heat can have on plant structure and development. Heat zones are defined as the average number of days, in a defined geographic region, above 86 degrees (F), the point when cell wall breakdown can occur that inhibits protein formation. It sometimes appears as browned or curling leaf tips, a loss of chlorophyll in plant extremities and bud or flower drop.  The higher your heat zone, the more days you can expect to occur above 86 degrees.

Looking at the heat zone map developed by the AHS, it seems that the Grande Ronde valley, AHS heat zone 5, will usually average 30 to 45 days per year at or above the 86-degree mark. This year may be the exception.

The AHS website,, has detailed information regarding heat zones and what prolonged heat does to ornamental and gardening plants.

To find heat zone information on their website, click on “Gardening Q&A” in the left hand column. A copy of the map is available for viewing on their web site and can also be purchased.

The important information to share here is that when our day heat is increased, so will the amount of water the plant needs to replenish loss through transpiration. Sometimes simply watering twice on a hot day isn’t enough, transpiration loss becomes too great and the plant suffers because of it.

If at all possible during days of high heat, such as Sunday was with the official high of 99 degrees, misting your garden plants that are most sensitive to water loss can assist them with survival and decrease the amount of heat damage that occurs. Yes, it is labor intensive, but the rewards are worth it.

If you aren’t able to visit our local Oregon State University Extension office and pick up free gardening information, a trip to the Extension website is worth your time. A multitude of articles and handbooks are available for viewing and printing.

If you are interested in drying fruits or vegetables this year, look at their online 32-page booklet titled “Drying fruits and vegetables,” second edition. Well-written and well-illustrated, this booklet is a treasure of food preservation information.

If you take the time to browse their entire site, you’ll find enough gardening information to keep you busy through the upcoming winter months. The Extension office site can be found at

Our sad peaches are ripening, sweet and juicy, but suffering tremendously from the hail damage of more than a month ago. Most of the peaches were at least half destroyed by deep pock marks where the dime-sized hail gouged the flesh.  While still good for fresh eating, they are ugly to look at and difficult to salvage. Maybe better next year?

Until next time, hope your gardening is good.

Jennie Lu Hagen is a La Grande gardener.