Plant Hardiness 

             Hardiness maps with hardiness zones are useful guides that are based in part on the lowest winter

 temperatures (in central Michigan that is usually -10 to -20 degrees Fahrenheit).

             For landscape use, some plants may be hardy in a certain zone, but their flowers may often be

 badly injured by frost.

             When flower buds and leaf buds start to swell or open, they are much more temperature

 sensitive than when they were dormant.

             Every garden has a number of microclimates. For example, a northern exposure that does not

 receive afternoon sun will be cooler and moister than a southern exposure. A northern exposure is also

 more susceptible to frost.

             Frost pockets are created when cold air flows down slopes and reduces temperatures at the

 bottom of the slope. Plants growing on higher ground have a better chance of survival.

             You can also use trees and shrubs to create microclimates for other plants, such as providing

 shade or wind protection.

             There are two main hardiness zone maps that are in use in reference books, the Arnold

 Arboretum Hardiness Zone map and the USDA Hardiness Zone map. In Michigan, we are in Arnold

 Arboretum Zone 4 and USDA Zone 5. For class, we are using the USDA Hardiness Zones.

References using the USDA Hardiness Zones:

1. Dirr, Michael A. l998 (5th ed.). Manual of Woody Landscape Plants

2. Still, Steven M. 1994 (4th ed.) and earlier editions. Manual of Herbaceous Ornamental Plants.

3. Flint, Harrison L. 1997(2nd ed.) and 1983 edition. Landscape Plants for Eastern North America.

4. III: Bailey, Liberty H. 1976 (by staff of the L.H. Bailey Hortorium). Hortus Third.

5. RHS: Huxley, Anthony, ed. 1992. The New Royal Horticultural Society Dictionary of Gardening.

6. Taylor's Guides (except Vegetables and Herbs).

7. HP Book on Trees and Shrubs.

8. HP Book on Bulbs: Scott, George Harmon. 1982. Bulbs: How to Select, Grow and Enjoy.

9. Jelitto, Leo, and Wilhelm Schacht. 1990(3rd. ed.). Hardy Herbaceous Perennials. 2 volumes.

10. Krussmann, Gerd. 1984. Manual of Cultivated Broad-Leaved Trees and Shrubs. 3 volumes.

11. Krussmann, Gerd. 1985. Manual of Cultivated Conifers.

12. Voigt, T.B., Betty R. Hamilton, and F.A. Giles. 1983. Ground Covers for the Midwest.

References using the Arnold Arboretum Hardiness Zones:

1. Dirr, Michael A. l990 (4th ed.) and earlier editions. Manual of Woody Landscape Plants.

2. HP Book on Perennials : Harper, Pamela and Fred McGourty. 1985. Perennials.

Park’s Books:

             3. Foster, Gertrude B. and Rosemary F. Louden. 1980. Park's Success with Herbs.

             4. Reilly, Ann. 1978. Park's Success with Seeds.

             5. Schneider, Alfred F. 1981. Park's Success with Bulbs.

Other Types of Zones:

1. HP Book on Annuals uses average last Spring frost dates: Zone 6 (May 10-May 20) = USDA Zone 5.

2. AHS (American Horticultural Society) Heat Zone Map (Aug. 1997) has 12 Zones based on the

 number of “heat days” which are days when the temperatures are over 86 degrees Fahrenheit

(30 degrees Celsius). This is the temperature at which many plants begin to suffer physiological damage

 from heat.

3. Growing Degree Days is another way to look at plant hardiness. Scheduling planting and

 harvest can on this basis can be used for many crops such as peas, beans, and sweet corn. Maturity dates

 of Concord grapes in Michigan can be determined based on this concept.

             Insect activity has been related to heat units.

             The amount of heat necessary for plant growth differs between species. Some crops require

 relatively warm temperatures , while others grow and develop in relatively cool temperatures.

             The temperature at which a crop begins to grow is referred to as the threshold or base temperature.

 Growing degre days are computed for base temperatures like 40, 45, 50, and 55 degrees.

             (1). First obtain the daily mean or average temperature by averaging the maximum and minimum

 temperature for the day (Ex. 86 + 66 =152, then divide by 2 for 76 mean temperature).

             (2). Then subtract the base or threshold temperature for the crop from the mean temperature. In

 our example, that would be (Degree days at base 50 deg.= 76-50 to = 26 growing degree days for that

 day).

`