Useful Guides to What
You Can Grow
The USDA Hardiness Zone
Polk County is in the USDA hardiness zone 9A. The hardiness zone is also known as the climate zone or growing zone. Zones range from zone 1 (Fairbanks, Alaska) to zone 11 (Honolulu, Hawaii). Basically, plant hardiness zones are a guide to help you know which plants will grow where you live, so you don't plant things that will soon die just because they can't manage the temperatures. It is based on average annual minimum temperatures. Plants vary in the temperature extremes they can endure. The zones for a plant are usually given as a range. For example; the zones for Bougainvillea are given as zones 9 to 11. This is the range for central Florida down to the Keys. In north Florida (zone 8) some varieties of this plant will freeze back to the ground and then re-grow from the roots. Some or your old favorite plants will not grow here because it does not get cold enough for a long enough period of time. Examples are lilacs, tulips, many types of lilies and iris, and most varieties of apples. What does this mean for the gardener?
- You should always check the Hardiness zone when purchasing plants. Choose plants where the range includes zone 9A.
- If you have the right micro-climate and are lucky, you can grow some zone 10 plants here.
- You probably will not be able to grow a zone 8 plant unless the plant's range includes zone 9A. It will not get cold enough most years.
American Horticulture Society Heat Zone
The American Horticulture Society has a new guide to help us grow plants. They call it the Heat Zone. It is based on the number of days a region can expect to experience temperatures over 86 degrees F (30 degrees Celsius). That is the point at which plants begin suffering physiological damage from heat. Just like the USDA Hardiness zone, this index can be used to help us choose plants that will grow in our climate. Zones range from 1 to 12. The Society hopes that over time, growers will mark more and more plants with the heat zone. Polk County is in heat zone 11 (180 to 210 days above 86 degrees). This is a relatively new index and has not caught on widely throughout the industry. It is difficult to find the heat zone for many plants. The best source is the American Horticulture Society web page (www.ahs.org under Publications. AHS has also published a reference book; the AHS Great Plant Guide, which lists over 3,000 plants and their heat zone and hardiness zone ratings.
The USDA Hardiness Zone is a measure of a plants tolerance and need for cold weather. The American Horticulture Society Heat zone is a measure of tolerance to hot weather.
Your Yard may be Different
Your yard may be just a little bit warmer than someone else's a mile away or even 100 yards away. It may be only a few degrees warmer during a freeze, but that few degrees can be the difference between a plant living or dying. These differences are called micro-climates. Cities tend to be a bit warmer than the country side. So if you live in the city, your plants may not feel the effects of a frost, that a friend's plants feel out in the country. Living on or near a lake can also make a difference. Differences can even be seen in a single yard. If an area is sheltered from the wind, near a lake, or even a foot or two higher, it can be a little bit warmer. The south, east and west areas of a yard are often just a little warmer than the north side. What does this mean to the gardener?
- A degree or two can make a big difference.
- They only way to understand your micro-climates is by experience. Some plants such as the common hibiscus (Hibiscus rosa-sinensis), hardiness zone 9-10, can be used as an indicator