Margaret Lauterbach

Thinking of becoming a gardener? Here’s a primer for southern Idaho

Margaret Lauterbach has the answer: What gardening zone are we in?

Idaho Statesman gardening columnist Margaret Lauterbach gives the definitive answer to the pesky and perennial question: Which zone are we in? Plus she has a few key pieces of advice specific to Boise gardeners.
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Idaho Statesman gardening columnist Margaret Lauterbach gives the definitive answer to the pesky and perennial question: Which zone are we in? Plus she has a few key pieces of advice specific to Boise gardeners.

Monrovia, one of the nation’s top grower of premium garden plants in the United States, reports at least 6 million more people became gardeners last year, at least 80 percent of them millennials. The gardening boom, especially coming from that generation, is due to gardening serving as a respite to a stressful world (I think) and a reaction to seeing colorful plants, especially ice cream colors, in nurseries (Monrovia thinks). There’s also a strong interest in soil health and sustainability as well as production of one’s own nutritious food in that health-conscious generation.

For several years I’ve received garden questions from as many young men and women as I’ve received from older readers, male and female. Those new gardeners here, along with a rapidly growing population of people moving from larger cities and other states to this area, need some basic information. Some of the newcomers are westerners, familiar with alkaline soil and diligent water conservation, but those from the Midwest and East are accustomed to acidic soils needing fireplace ashes or lime to sweeten the soil as well as abundant natural water. Our soil is already sweet, so don’t add lime or ashes.

If you have spread either lime or ashes, try to remove it, but if you can’t do that, build raised beds and use different soil to fill them. Incorporating an abundance of organic matter has reduced the alkalinity of my garden soil to about 7.0. Some soils are even more alkaline. Soils with a pH of 7.5 and higher are too alkaline for nearly all plants.

This alkalinity is measured by pH, technically hydrogen potential, and the higher the number, the more alkaline. Our water tends to have a 7.0 pH, a bit on the alkaline side for plants, since most plants prefer a pH of about 6.5 to 7.0 for soil and water. If the pH is off, plants struggle and die since they cannot take up the nutrients they need.

Our average annual precipitation is 12 inches, with low humidity. This is a steppe climate, near desert. Some residents of the Valley receive irrigation water from the canals, but the rest of us either have private wells or must use city water for our lawns and gardens. City water is not cheap, so soaker hoses and drip systems are ubiquitous. Most of our garden plants require an inch of water per week, and even more when the daily high temperature is running over 100 F and they will not get that much from rainfalls. We usually have only one or two days of rain from June to September.

When you’re looking to buy shrubs, vines, trees or perennials, watch for hardiness indication on the plant label. Our USDA hardiness zone is officially zone 7, but you’re safer to assume it’s zone 6 since we’ve had zone 6 winters the past two or three winters since USDA raised our zone to 7. Zone 7 means expected winter low temperature does not drop below 0 degrees F., but it has dropped below zero the past few years.

If you plan to put your new plant in a container and do not plan to bring it into a garage or greenhouse for winter, only buy a plant that’s hardy to zone 4. Plants in ground have the added protection of the surrounding soil insulating them from severe cold, but those in containers have little soil plus the composition of the container to protect them.

Because of this climate, we have a hardpan in our soil at various depths, especially south of the Boise River. This hardpan is called caleche, and is equivalent to a 2-inch thick layer of concrete. In my area near the airport terminal, caliche is 30 inches below surface, but properties further west find it’s closer to the surface. If you’re growing taprooted trees, break up the caliche before planting.

With care, you can grow acid-loving plants here. Some folks plant blueberries in pure peat moss, for instance. Azaleas and rhododendrons must be fertilized with acidic fertilizer such as Miracid or Hollytone. If you are set on growing bog plants, buy a child’s wading pool, dig a hole so that the pool can be set into it to its brim, then poke holes for drainage and fill the pool with soil, peat moss and water (watching out for the pH in the mix of soil and peat moss).

Another garden aspect new residents should be aware of include watching for blossoming of forsythia. At that time, you can safely (not risk freezing damage) prune your roses and/or apply crabgrass preventive to your lawn. Crabgrass germination requires as many chilling hours as forsythia needs to bloom. On this topic, you can force earlier blossoms of forsythia or fruit trees as cut flowers by cutting branch tips for indoor vases, and putting them in warm to rather hot water. Change water daily.

Once you’ve pruned those roses, begin to fertilize them once a month, until Aug. 15. Stop at that time to let the shrub winterize itself. Some rose shrubs do develop hips (seed cases) while winterizing. Do not prune any spring-blooming shrub until after it blooms, and in any case, don’t prune anything if rain is in the immediate forecast. Splashing water often carries water-borne disease bacteria and viruses.

Trees begin to shut down for winter about June 15, so do not fertilize them after that date.

The aridity of our climate makes weather comfortable to humans and protects our plants from many diseases. We seldom have such wet springs that our stone fruit trees develop peach leaf curl, for instance. Watch fruit trees and ornamentals such as pyracantha, for signs of fireblight. That disease may attack any member of the rose family, including hawthorns, mountain ash and spirea as well as apples, pears and of course roses. We usually don’t have problems with blights (early and late) in the food garden either, although we do have damaging curly top virus that is not present in more water-abundant areas.

Our last average frost date is May 10, but if you have frost-tender annuals or perennials to transplant, it’s safer to wait until closer to June 1 for planting out. Our average date of first frost is about Oct. 10, so we have about a five-month growing season. Note that frosts may be earlier or later than average. We generally feel winter is over when the last snow on Shafer Butte has melted. We can see that from the Valley.

Send garden questions to or Gardening, The Statesman, P.O. Box 40, Boise, ID 83707.

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