Green thumb or not, you’ve most likely seen the evidence all around—Camellias, have bloomed, Daffodils are up, lettuce is sprouting; flowering trees have blossomed, others have buds on them—all significantly earlier than in years past. And it’s not only the present winter that has been mild, looking back to last fall, the growing season for vegetables extended much longer than normal. Leading to one big question for farmers and gardeners: Has our growing season changed? And an even larger one—if so, does it suggest real evidence of climate change brought on by global warming?

Most gardeners would say that you couldn’t make a blanket statement based on one year’s milder-than-usual weather patterns. But, scientific evidence does seem to substantiate that the climate, and therefore the growing seasons, have indeed changed.

The USDA has just released a new Plant Hardiness Zone Map. Divided into 13 sections based on the average lowest winter temperatures in various areas of the U.S.. These zones are what professional and home gardeners and farmers refer to when determining what plants to put into the ground, and when. On average with the new map, most areas have moved up half a zone indicating warmer weather, with the exception being a few areas of higher elevation in the west, that have moved down a half zone suggesting colder winters.

This map is the most sophisticated ever created by the USDA. It takes a number of factors into consideration—weather measurements, global positioning, elevation, winter temperatures, close proximity to a large body of water, the urban heat island effect—meaning that it isn’t easy to pinpoint one exact cause of what appears to be a warming trend, global warming or otherwise.

For practical purposes, the zone re-mapping doesn’t equate to significant change; all plants that were hardy in the old zones will transfer to the higher zones, with new additions. And yes, in some cases, planting could occur earlier. But, milder weather and new zoning doesn’t necessarily mean it’s time to start putting seeds and small plants in the ground.

Ellen Nabali, a certified professional horticulturist with the University of Maryland Extension Home and Garden Information Center, indicated the safe bet is to wait until the ground itself is warm and dry enough to provide the best possible environment for plants to thrive.

“It does seem to be a general progression towards warmer weather, but the ground is still cold. If you put plants in too early they won’t do well. And the ground is still very wet—gardeners just can’t work with wet soil,” said Nabali.

For those who wish to get outside and take advantage of the warmer weather, Nabali suggests pruning trees and weeding.  

“Weeds germinate in the fall and grow all winter—get them out now,” recommends Nabali.

Gardeners who have questions regarding the new zone changes, what to plant when, or any other query that could benefit from expertise like Nabali’s, can call or email The Home and Garden Center through the University of Maryland Extension. Certified horticulturists respond to gardening questions for Maryland citizens, (and those from all over the country, really) free of charge.

Nabali was asked a very specific question regarding the perceived change in the growing season, based on the behavior of tomato plants last fall. Heirloom tomato plants turned into vines spanning over ten feet, and continued to produce fruit into December, when the first cold spell hit. Was this strange behavior the result of the weather, a mutation of the plants themselves, or a bizarre and unexplained situation?

Turns out, tomato plants are either determinate or indeterminate. Those that are indeterminate (like many heirloom varieties), will continue to grow in climate’s that support them. Such was the case brought on by last fall’s warmer than usual weather, until the first frost.

This behavior isn’t a mutation, but rather typical for certain tomatoes including the heirloom, cherry, beefsteak, and Brandywine varieties. Many gardeners don’t allow their plants to actually vine. If you do, the plants must be supported because they can get quite heavy, especially during a lengthy growing season. The best option for vining involves hanging the plants, allowing light to reach all parts. If mild weather patterns continue, the growing season for tomatoes could once again extend into the fall and early winter.  

Farther south, gardeners are lopping off the tops of tomato plants and bringing them indoors to over-winter during the few months of harsh, cold temperatures, creating seedlings from cuttings and extending the life of the plants. Although due to the colder climate, this practice of cutting and extending isn’t an option for Eastern Shore residents, yet.

More information on the University of Maryland Extension Home and Garden Information Center, and the USDA new hardiness zone map can be found in Gardening Resources below.

Gardening Resources:

USDA New Hardiness Zone Map: Enter your zip code for an exact zone for your location.

University of Maryland Extension Home and Garden Information Center site

Phone calls: 800.342.2507 or 410.531.1757

  • Gardeners are also encouraged to email photos of problems they are having, or even strange and unidentified plants they encounter, for a proper diagnosis, solution, or identification. Questions can also be asked via email.

UMD Extension Home and Garden sister site all based around how to grow and care for vegetables, Grow It Eat It.