By now you have likely heard that in November 2023 the U.S. Department of Agriculture updated its plant hardiness zone map for the first time since 2012. Part of Duval County from around the St. John’s River and east changed from Zone 9a to Zone 9b. You may be wondering what it means for you, your garden and edible crops you grow. If so, read on for answers to specific questions you may have!
What does a change to Zone 9b mean for my vegetable garden?
For vegetable gardening, we always recommend right plant, right place, right season. That means we grow our most cold-tolerant crops in winter when we have our coldest lows. These include the Brassicas like collards, kale, cauliflower, broccoli, turnips and mustards. Other good cold hardy crops are lettuce, carrots, celery, onions and strawberries. Some of these you may need to cover or harvest before a hard freeze, but most will sail through a short period of temperatures just below freezing quite well. If a longer, deeper or multiday freeze is predicted, then you will need to cover even the most cold-tolerant vegetables. The change to Zone 9b might just mean there are fewer days (on average!) that you need to worry about covering your crops. If you are trying to nurse along some fall tomatoes or hot peppers, the change in zone won’t matter. Either way, the extreme lows are too cold for most of our warm season crops. Because we will still have frosts and freezes in Zone 9b, we still won’t likely be able to grow more tropical crops except as annuals during our warm season.
How will the change affect fruit trees and shrubs?
Slightly warmer winter lows likely won’t affect most fruit trees we grow. What might be detrimental to more temperate zone crops like apples, pears, peaches or blueberries is a decrease in chilling hours. The Plant Hardiness Zone Map does not address any potential changes to chilling hours. As Julio Perez, Horticulture Extension Agent in Putnam County explains, “Chill hours…represent the seasonal accumulation of temperatures below 45 degrees F, crucial for certain trees and shrubs—especially fruit-bearing ones—to break dormancy and flower. When selecting fruit tree varieties, it’s essential to consider information from both factors to ensure they survive and produce fruit.” If the tag on a nursery tree does not state how many chilling hours are required for it to flower and produce fruit, you can often find that information on a reputable nursery’s website. Chilling hour requirements can vary significantly between varieties. In general, look for low chill varieties. For more information on chilling hours and fruit trees, visit this blog from our Extension friends in the Panhandle.
For up-to-date information on chill hour accumulation near Jacksonville, see AgroClimate’s Chill Hours Calculator. The closest monitoring station to Jacksonville is in Baker County. As of mid-December 2023, it logged 96 chilling hours. The average chill hours per year for us in Duval County is between 540-660 hours. Be aware that chilling hours in the city are typically lower than those in more rural areas because of urban heat island effects.
Does a change in Zone impact insects?
It may. As minimum temperatures warm, certain plants and insects may expand into areas where they previously wouldn’t have survived the coldest winter temperatures. This could mean that vegetable insect pests like whiteflies become more problematic during our warm season. Why? Whiteflies typically are not able to overwinter in large numbers in north Florida. They move north each season from south Florida, which is why they are typically more abundant in the late summer and fall here. The change to Zone 9b may still not allow large populations of whiteflies to overwinter here, but much of the Orlando area is now Zone 10a, meaning their extreme lows (30-35°F.) may be above freezing. Whiteflies may have an increasingly short distance to go when repopulating our north Florida gardens. So, whiteflies and the damage and disease they cause may show up earlier in our growing season.
Does the change to Zone 9b mean I can plant my tomatoes earlier?
No. The Plant Hardiness Zone Map is based only on extreme minimum temperature averages. These occur when we are not growing tomatoes, so the change in hardiness zone does not affect planting dates for warm season crops. The average last date of frost for the winter season determines when we typically plant our warm season crops outside. You can find data from the National Weather Service on Jacksonville’s first and last frost dates online. For the period 2000-2023, our average last date of frost was Feb. 24. The earliest was Jan. 22 in 2020; the latest was Apr. 8 in 2007! There’s lots of variation from year to year, so pay attention to local weather, and check and keep records of temperatures in your own garden.
Article written by Beth Marlowe, Urban Garden Program Assistant, February 2024
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