Before you start your own home vegetable garden, there are a few factors to take into consideration before ordering vegetable seeds. These are your USDA hardiness zone, your AHS Plant Heat Zone, and your microclimate.
The United States Department of Agriculture (USDA) provides the US with a zone hardiness map as a general guideline for what will grow in each gardener’s particular area. It’s important to understand that these are very general guides. No map can guess about specific microclimates that may be affecting a city, neighborhood, or yard.
Some people argue that the USDA hardiness zone map is used when discussing perennial plants, trees and shrubs. In reality, the zone map is the the most helpful for those plants, but knowing your zone will give you some clues to which vegetable varieties to choose. Any annual can be grown in any zone as long as it provides enough warm days for any individual plant variety to reach harvest. This is why it’s important that gardeners find out the days until harvest (or get an approximate count) for varieties before purchasing vegetable seeds.
As an example, for a vegetable that requires 100 days of warm weather planted in an area with an average of 120 days of warmth or more, this would work. However, if it’s discovered that the growing zone has only 60 warm days, it would be necessary to choose a variety that has a shorter harvest time.
To find a particular zone on the hardiness map, the gardener must first find out what the minimum temperature is in their area (the local Cooperative Extension office may be contacted, or weather station if this is unclear). If the area dips down to 20 – 30 degrees, this is zone 8; below 40 degrees but above 30 degrees? It would be zone 3. If the temperatures only drop as low as 50 or 60 degrees, this would mean the gardener is in Hawaii or somewhere similar; zone 11.
Another general guide, which may be more helpful than the USDA map, at least in the case of annual vegetables, is the American Horticultural Society’s Plant Heat Zone Map. This guide is based on the average highs instead of the lows of plant survival. Here the average number of days that temperatures are 86 degrees and above is indicated.
Once the zone number is established, the AHS map will tell gardeners approximately how may days that area has for some nice growing weather. In zone 7, there are about 60 to 90 days of very warm temperatures. The gardener should then choose vegetable varieties that mature within that time frame – or at least close to that time frame. The temperature can be pished for a little longer (or shorter) amount of time with microclimates.
Microclimates are those small areas that are different than the larger general region. They may be created by a number of things such physical structures, extra windy areas, topography, or large bodies of water. For example, a gardener may find that their zone is said to be prone to heavy frost, but become confused when their plants rarely succumb to an icy death.
There can be many reasons that a gardener’s “zone” doesn’t act as it seems their zone should. In urban areas buildings (or lack thereof) can have a huge effect on the climate. They may acts as wind barriers or completely the opposite; creating wind tunnels. If there’s a large body of water nearby such as a lake or the ocean, this tends to moderate the air temperatures of nearby inland areas.
Topography certainly plays a major role for microclimates. Living on top of a hill or deep in a valley plays major roles for microclimates. Warm air is lighter than cold air, so if the gardener lives in a valley they may have more frost problems than someone living higher up. On which side of the hill does the gardener reside? A northern slope is slower to warm up, but a south-facing slope is a mixed bag. The southern side warms up faster, but if plants begin to bloom, they could be set back if a sudden frost hits.
Other factors that can affect microclimates are rainfall, soil types, mulching practices, paved surfaces, fences, walls, raised beds, cold frames, balconies, and rooftops. So if gardeners think that the numbers on a map have the last word – think again. The cleverest gardeners will manipulate their microclimate by using any number of the above resources to their advantage.
For more on growing vegetables, check out Grow Your Own Garden Vegetables, Easy to Grow Vegetables for Your First Garden, and What Compost Does for the Vegetable Garden.
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