This past winter was one of the warmest and wettest on record.

What do these weather extremes mean for the plants in your garden?

The answer isn’t simple. One tool to help is the U.S. Department of Agriculture’s plant hardiness zone map. The agency updated the map in November for the first time in more than a decade. About half of the country saw a shift and started the 2024 growing season in a new gardening zone.

Here’s more about the changes from USDA and Penn State Extension.

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What is a hardiness zone?

Hardiness zones show where plants are likely to thrive. There are 13 zones based on average lowest temperatures. Zone 1 in parts of Alaska is where average low temperatures reach minus 60 to minus 50. Zone 13 in parts of Hawaii and Puerto Rico has low temperatures from 60 to 70 degrees.

Full zones are 10 degrees apart. Half zones (A and B) are 5 degrees apart.

What zone is Lancaster County?

Most of Lancaster County now is in Zone 7a, with a small segment between Martic Forge and the Susquehanna River in 7b.

What changed in the new map?

Lancaster County communities moved into warmer zones from the 2012 map to the 2023 map. For example, Lancaster moved from 6b to 7a. In 7a, the average low temperature over 30 years was 0 to 5 degrees.

Why did the zones change?

The map is based on the average low temperature over 30 years. Over the latest 30-year period, average low temperatures in Lancaster County increased by 4 degrees, which shifted the zones. Nationwide, the average low temperatures were 3 degrees warmer.

This isn’t the first shift. Plant hardiness zones across the country (at least the lower 48 states) have moved north for the last three updates as temperatures warm.

The new map also has data from twice the number of weather stations.

What does this mean for plants?

You don’t need to change the plants in your garden immediately. “What has thrived in your yard will most likely continue to thrive,” according to USDA.

A longer growing season opens up the possibility of growing something more tropical. Consider canna lily bulbs, which usually need to be dug and stored to survive Pennsylvania winters. Lately, local gardeners noticed forgotten bulbs survived the winter and bloomed in the spring. The same goes for plants such as pansies, snapdragons and dusty miller.

However, some plants need cold temperatures in the winter to blossom.

Temperature shifts can also change when insects arrive, including beneficial insects like pollinators and harmful pests, plus invasive plants like kudzu.

READ: Garden guide 2024: 9 beautiful plant places in the Lancaster County region (and some are free)

Can I keep my plants out later in the fall and plant earlier in the spring?

No, unless you want to be a weather watcher.

The hardiness zone is based on average yearly low temperatures. Nights when temperatures dip below the average can still damage plants, especially tender plants grown as annuals such as tomatoes and houseplants. If tender plants are outdoors, watch for low temperatures and protect by covering or bringing indoors.

The map also doesn’t take into account the hot temperatures, which can hurt plants. Watch the temperatures to add shade and extra water on the hottest days.

What else is helpful to understand your garden?

Microclimates are areas that can be warmer or cooler than the surrounding space. For example, a hot spot next to a paved driveway or a low spot where cool air collects. Pay attention to these areas, especially for plants suited for much warmer or cooler climates.

Ecoregions also describe ecological areas and cover more than temperature, including precipitation, soil type and wildlife.

Can I grow citrus and palm trees?

Not outdoors year-round, but you do have options. Seeing average low temperatures that are a few degrees warmer does not mean Lancaster County is a tropical rainforest.

If you want to grow citrus or palm trees, local greenhouses have a range of varieties to grow outdoors in the summer and indoors when things turn chilly.

Growers at Tyler Arboretum in Media have a smaller experiment. They planted their earliest crop of the year in February: carrots and beet seeds in soil hills protected by a wind tunnel and wait to see what happens.

For plants expected to last generations, take a longer view.

“If you’re putting in a plant, like a tree, that going to last a long time, you’ve really got to think about climate change,” Ray Najjar, climate scientist, professor of oceanography in Penn State’s College of Earth and Mineral Sciences, told a group of master gardeners in February. “If you’re planning for next year’s garden, the last year is a good indicator of what’s going to happen in the following year.”

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