Get in the Zone

Moving to Montana from warmer climes and opening up the Bed and Breakfast which has a greenhouse, this has been an important factor in our planting cycles.  But the gardening zones, are actually misunderstood by lots of folks in terms of what they're able to grown, and when.

The USDA zones break the United States into 11 individual zones. There is a 10° difference in zones. Each zone tells us the annual average coldest temperature in that area. Since the coldest temperature happens in the winter, zones have nothing to do with the crops you grow only in spring, summer or fall.

Zones don’t tell us when to expect our last spring frost or first fall frost. Those dates vary by several weeks throughout the zones. A colder zone than the one you’re in might have an earlier last frost date, allowing you to plant tender annuals ealier. The other zone has a lower average temperature in the winter but it warms up sooner in the spring.
Zones don’t tell us the length of daylight, the average high temperature, the number of days above a certain temperature, or what can be planted at any given time.

So when do we need to use zones? If you farm in warmer climates you need to know how much cold a plant must have. For example, some fruit trees need a period of dormancy over the winter before they’ll blossom and produce again. If you’re in a zone that doesn’t get cold enough to provide this rest period you’ll have a harder time growing those fruits. The tree will most likely survive but not produce.

If you're growing perennials in a cold climate you need to know what zone those plants are hardy in, and in which zone you're located. Perennial vegetables such as asparagus, artichokes and rhubarb are hardy enough to survive winter in some zones but not in others. I grow artichokes as an annual because they won’t survive at -25°F. We need to know what zones perennial herbs are hardy in. Perennial herbs in warmer climates are annuals in the colder climates. When you’re farming in the of warm and cold you can mulch some herbs heavily in the fall and get them through winter. Flower bulbs will survive very cold winters and break dormancy as soon as the soil starts to warm, sometimes before all of the snow is gone. If the same bulbs are planted in a warm zone the plant is likely to have a lot of foliage but poor flowering ability.

The Zones
There are 11 numbered zones that break down into lettered zones. They start at zone 1 and go to zone 11. Zones 2 through 10 are divided into a and b zones with 5° differences.

1 is Below -50° F
2a is -50° to -45° F
2b is -45° to -40° F
3a is -40° to -35° F
3b is -35° to -30° F
4a is -30° to -25° F
4b is -25° to -20° F
5a is -20° to -15° F
5b is -15° to -10° F
6a is -10° to -5° F
6b is -5° to 0° F
7a is 0° to 5° F
7b is 5° to 10° F
8a is 10° to 15° F
8b is 15° to 20° F
9a is 20° to 25° F
9b is 25° to 30° F
10a is 30° to 35° F
10b is 35° to 40° F
11 is Above 40°F

Microclimates
The USDA zone map doesn’t guarantee a definite average minimum temperature. Microclimates are small areas inside a zone that are a little warmer or cooler than the surrounding area. There are factors to take into consideration. Hills, valleys and windbreaks change the flow of air. A change in air flow can cause warmer or cooler air to be trapped in an area, or move out around that area. Buildings will absorb heat during the day and release it into the evening and night (radiant heat), keeping that small area a little warmer.

One of the joys of having a greenhouse is the ability to grow fruits and veggies year round that wouldnt otherwise like being "in the zone".