Climate zone info is critical in plant selection
Q
Could you please explain climate zones and how important it is to select plants appropriate for your climate zone? When I shop for plants online, I make sure the plant will grow in Zone 9 (my zone) but I'm wondering if that matters, as it seems that the zone pertains to cold hardiness, which isn't an issue here in Southern California. Does the zone number also pertain to humidity or moisture conditions? How far from my zone could I extend and still have success with my plant, assuming I give it extra care: Zone 8 or Zone 7?
For example, I have heard that you can place ice cubes around a plant to simulate harsher climates. Also, is there any value to knowing the Sunset climate zone, which is much more specific? Plant retailers do not list the Sunset zone so I'm not sure what I can do with this information. Finally, how can we identify microclimates on our property and determine whether we can use plants appropriate for a different zone?
A
The 13 U.S. Department of Agriculture climate zones are based on the average winter low temperature for a defined region. The coldest zone is 1a, with an average winter low temperature of minus 60, and the warmest is Zone 13b with an average winter low of 65. Each zone is separated by 10 degrees, and the “a” and “b” represent a 5-degree difference. For instance, Zone 13a has a winter low of 60, and Zone 13b has a winter low of 65.
These zones apply across the entire United States and can help you decide whether something is likely to survive winter where you are. They can also provide information as to when to plant cool- or warm-season annuals (particularly vegetables). They don't tell you what the first or last frost dates are, as these can vary between areas that have the same zone designation.
• USDA Plant Hardiness Zone Map
The Sunset Western Garden Book uses its own zone designations, and since it covers only the Western United States, is much more detailed and useful (well, at least for those who live in the Western U.S.). Sunset zones describe not only the average winter low temperature but the length of that region's growing season, amount and time of rainfall, average high temperature, wind and humidity conditions.
• Climate Zones, Sunset Western Garden Collection
Sometimes, if you are in a marginal zone, you can use microclimates to your benefit. For instance, some frost-tender plants can be located near a south- or west-facing wall for a few degrees of frost protection. An additional consideration is winter chill hours. Many cold-area plants will grow in a warmer zone but will decline and die within a few years if they don't get enough winter chill. This is especially true for certain fruit trees, such as apples, pears and cherries. A simple definition would be the total number of hours below 45 degrees. Think of it as a plant's nap time. This can't be reproduced by putting ice over the root zone.
You can find more information by searching “winter chill” at climatehubs. usda.gov.
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Coleus care and feeding: The colorful foliage of coleus is a perpetually welcome sight to see, a mini-caladium experience. Coleus thrives throughout the growing season and may even persist into winter. The plant is somewhat tricky to grow since it burns in too much sun and flounders in too much shade, so filtered sun is best. Since its roots are shallow, you will also want to keep it away from trees and shrubs whose surface roots could interfere with its own. You may see some blue flowers begin to appear this time of year. It is advisable to remove them since, underwhelming in appearance, they will nevertheless halt the growth of the colorful foliage for which you planted coleus in the first place. Keep coleus happy with fish emulsion, which contains only 4%-5% nitrogen, or another low analysis fertilizer. An incalculable bonus of growing coleus is its ease of propagation. Cut a shoot, remove its bottom leaves, place it in a glass with a little water, and it will begin to grow roots this time of year within a week or two at most. Vinca easy to love: Annual vinca (Catharanthus roseus) thrives in hot weather. It has a proclivity to die young when planted in poorly drained soil, and care should be taken never to overwater it. Yet it blooms vividly in pink, red, apricot, mauve and white, and is worth the risk. Although widely considered to be an annual, I have seen it live for many years where soil drainage is good and it is watered barely more than if it were a cactus. Where this plant is long-lived, it also self-sows, so new plants are always coming along. Ideally, you will water it by drip irrigation or carefully from a hose to make sure its stems and leaves stay dry. It will fare well in flower boxes when planted in a cactus soil mix, although you will have to water more frequently than if it were growing in the ground. Vinca minor is an excellent perennial ground cover for partial sun to shady exposures, with blueish-purple flowers. It forms a tight mat that eliminates evaporation of water from the soil surface and thus it is drought-tolerant once established.
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Rose and shine: Prime your roses for fall bloom. Now is the time to remove all dead stems and faded flowers, as well as any hips (fruit) that have begun to form. You can also lightly cut back the entire plant to encourage new growth and flower bud formation. Cut suckers that have sprouted up from below the bud union on your bushes, to be distinguished
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Protect your containerized specimens from drying out by placing them in larger pots. Let's say you bring home a plant in a plastic nursery container and want to put it on your patio. Procure a terracotta or ceramic pot of a slightly bigger size and place the plastic container inside it. The sun will strike the sides of the larger pot and the roots of your specimen in the plastic container will stay cool. If you are going away for the weekend and are concerned your plant could still suffer from the heat, soak some peat moss in water and stuff it into the space between your container and the pot around it. The moist peat moss will serve as a buffer, keeping the roots of your potted specimens cool. Of course, providing drip tubing, whose watering is controlled via automatic sprinkler controller, will allow you container peace of mind if you are away for an extended period.
Moving day: If your plant has outgrown its container, now is the time to move it to a larger size. While it is still actively growing, you want to make the move. As days get shorter in the approach to fall and the rate of growth slows, a plant in a new container runs the risk of being watered to no effect, and the roots may rot from excess moisture. The surest way to tell if a plant needs to be repotted is if roots grow out of the drainage holes at the bottom of its container. It is a good practice to cut away any dead or circling roots before repotting, while surrounding the root ball in its new container with fresh potting soil is recommended. You can remove as much as 1/3 of the root mass of a plant when repotting.