Micro-Climates and Hardiness Zones
Most backyard orchardists and gardeners know what their Hardiness Zone is. If they don’t, they should learn what it is.
The United States Department of Agriculture (USDA), using long-term weather and climate data, has broken down the U.S. into 13 Hardiness Zones, from Zone 1 (darn cold) to Zone 13 (dang hot). They vary by 10 degrees each based on the “Average Annual Extreme Minimum Temperature.” The data used in the most current edition is from 1976 to 2005.
If you are a backyard orchardist of deciduous fruit (apple, pear, cherry, peach, nectarine, apricot, and plum) you should know where these varieties will best perform and grow, usually in zones 4 to 8.
Over approximately a 20-year time-span, your location will have an extreme cold event that basically determines your Hardiness Zone. For example, if you are located in Zone 5, the coldest temperature you will “likely” experience in a 20-year period will be between -10 and -20F.
To look up your Hardiness Zone according to the latest and supposedly “best” edition of the map, click HERE
Climate change and zone migration
Grandpa is slightly skeptical about the maps, because they are based on averages over many years, and he feels that the latest one appears to be somewhat based on the theories that the climate is warming and the Hardiness Zones are “migrating” northward. Time will tell for sure, but another facet of the climate-warming theory is that there will be more extremes of temperature, so possibly this means that the winter cold extremes may become even worse than before, thus possibly negating the migration.
Regardless, the Hardiness Zones are useful in helping the backyard orchardist determine what varieties he should or should not be growing because of their sensitivity to winter cold. Most fruit varieties have zone ratings, however, these should be taken with a grain of salt. In some cases, the determination may just be an educated guess based on the genetics of the variety or even wishful thinking by the promoter. Since there doesn’t seem to be any other way to rate varieties, we must use them judiciously and carefully.
A and B zones
The current USDA map also has “A” and “B” zone location, which are there to more accurately define the edges and centers of the zones. This is in recognition that finer strokes of the brush are needed to paint the Hardiness Zones properly. This is useful information, but also must be taken with a grain of salt. By using the current USDA map(link found above), Grandpa finds that his nursery is in Zone 6B, which means that his average annual minimum temperature should be from 0 to -5F. However, on several cold mornings this winter Grandpa woke up and checked his digital thermometer and found -17F. Does this mean I live in Zone 5A? Well, not necessarily, I feel it’s not really an exact science.
Know your micro-climates
Micro-climates can be considered “mini-zones” within major Hardiness Zones. Many orchards and vineyards are planted in micro-climates because the growers know that certain sites will perform better than others. In general, excellent fruit sites have a much higher elevation than the surrounding areas. These sites are less susceptible to frost and freezing events (even in severe winters like this past one, where a clear cold night can cause way-below-zero temperatures in the early morning hours) because of cold air flowing downhill, in the same way water would. So, the higher elevations are usually warmer than the valleys. This type of micro-climate will not protect your plants and trees from the worst extreme cold events in your Hardiness Zone, but will be helpful in producing consistent fruit crops in the years ahead.
Micro-climates are also found in proximity to bodies of water that do not often freeze over completely. Lake Michigan is a typical example, moderating the cold blasts that come from the west (though it isn’t much help when the cold winds come from the east!). Historically, big lakes keep the winter temperatures warmer and the spring temperatures somewhat cooler than areas without big bodies of water.
If you are planting a backyard orchard, you should take account of your USDA Hardiness Zone and choose varieties that are rated for that zone first. You take a chance of losing, when you choose varieties that are not indicated as hardy enough for your zone. Next, identify the micro-climates on your property and use those to your advantage when taking time to consider what you will plant.
No matter how carefully you plan, however, you aren’t going to beat Mother Nature when she wants to throw something extreme at you. Fruit growing has always been a gamble, so you have to be prepared to roll the dice!