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What zone are you in?
Virtually all plants are cataloged according to the USDA zone map. In 2006, arborday.org redrew the zone map of the United States. Our nearest town, Coatesville, PA, used to be zone 6, and is now 7. In fact, much of southeastern PA was zone 6 and is now zone 7. They are basing this information on the fact that their have been very few temps below zero in the past decade. True.
But does this tell the whole story? I would suggest that an average of daily high and low temperatures would be more useful in determining the success of some plants, particularly evergreens. For example, Greensboro, NC used to be in the middle of zone 7, and West Chester, PA is now. However, look at the following charts:
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Average High Temperature Greensboro, NC |
Years on Record: 48 |
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YEAR |
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Jan. |
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Feb. |
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Mar. |
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Apr. |
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May |
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Jun. |
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Jul. |
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Aug. |
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Sep. |
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Oct. |
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Nov. |
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Dec. |
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°F |
69 avg |
48 |
51 |
60 |
70 |
78 |
84 |
88 |
86 |
80 |
70 |
60 |
50 |
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Average Low Temperature Greensboro, NC |
Years on Record: 48 |
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YEAR |
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Jan. |
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Feb. |
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Mar. |
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Apr. |
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May |
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Jun. |
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Jul. |
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Aug. |
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Sep. |
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Oct. |
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Nov. |
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Dec. |
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°F |
47 avg |
28 |
30 |
37 |
46 |
55 |
63 |
67 |
66 |
59 |
47 |
37 |
30 |
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Average High Temperature West Chester, PA |
Years on Record: 30 |
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YEAR |
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Jan. |
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Feb. |
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Mar. |
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Apr. |
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May |
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Jun. |
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Jul. |
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Aug. |
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Sep. |
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Oct. |
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Nov. |
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Dec. |
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°F |
63 avg |
38.1 |
40.7 |
51.3 |
61.8 |
72.4 |
81.1 |
85.7 |
84 |
77.4 |
66.3 |
55.3 |
42.9 |
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Average Low Temperature West Chester, PA |
Years on Record: 30 |
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YEAR |
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Jan. |
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Feb. |
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Mar. |
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Apr. |
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May |
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Jun. |
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Jul. |
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Aug. |
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Sep. |
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Oct. |
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Nov. |
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Dec. |
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°F |
41 avg |
19.3 |
21.2 |
29.9 |
39.3 |
49.3 |
58.5 |
63.3 |
61.5 |
53.9 |
42 |
34.4 |
24.2 |
The average of high/low temps in Greensboro in January is 38F, but West Chester is only 28.7F. The greater temperature rebound during the day in North Carolina doesn’t allow the ground to freeze as deeply as it does in Pennsylvania. The average hi/low temperature average hasn't changed very much in the past several decades. Therefore, marginal (zone 7) evergreens such as Loropetalum and Rhaphiolepis are still unknown in Pennsylvania, due to dessication while the ground is frozen, and perennials such as Canna, rarely overwinter unless generously mulched due to the freezing of the rhizomes. Planting in microclimates such as near a wall or building may help, but...
All this is to say – choose your plants and your site wisely and avoid replacements! I love pushing zones as much as the next person, but their are limits. So for practical purposes, we are still zone 6.
http://www.arborday.org/media/mapchanges.cfm
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