It's a bit early to directly transplant indoor seedlings, but it's the perfect time to start hardening them off in preparation for outdoor planting. Credit: Sam Schipani / BDN

There is nothing like early spring warm temperatures to lull a gardener into a false sense of security. Threats of freezing temperatures in parts of the state this week are snapping many would-be planters back to reality.

The National Weather Service on Wednesday issued a freeze watch for parts of Piscataquis, Penobscot, Kennebec, Oxford, Franklin and Cumberland counties through Thursday morning. Temperatures are expected to drop into the upper 20 degrees Fahrenheit, below freezing.

Among the precautionary actions the National Weather Service recommends is protecting outdoor plants and crops from the cold that could kill them. This could be done by covering plants or by bringing potted ones indoors.

That, said Kate Garland, horticulturist with University of Maine Cooperative Extension, is good advice to heed. She recommends that anyone thinking about putting seedlings or transplants into the ground this week should wait.

Maine typically sees the final frost of the year by the end of May, though there are parts of northern Maine that can experience a frost right up until the end of June. Recent warm temperatures around the state that hit the 70s over the last week in no way mean those final frost dates are going to be earlier this year.

“Back in 2020, there were some very chilly nights in early June that caught a lot of gardeners by surprise,” Garland said. “Years before that, I’ll never forget the disappointment of having my hardened off tomato plants get hit by a cold evening shortly after Memorial Day weekend. It wasn’t a full frost, but it was cold enough to really set them back to a point where I decided to replant.”

No matter how favorable conditions are in May, Garland recommends planting most warm-season crops like tomatoes, peppers and cucumbers the first week in June.

“Gardeners in the further north may want to wait another week to 10 days to be on the safe side,” she said. “More important than the specific date is the weather forecast.”

Even though it’s safe to plant cold-tolerant vegetable seeds like onions, broccoli, cabbage, lettuce, swiss chard or beets now, seedlings should be planted later.

However, Garland said now is the perfect time to start hardening off those seedlings.

Hardening off is the process of getting seedlings that were started inside acclimated to the outdoors by slowly exposing them to the elements. Seedlings transplanted directly to an outdoor garden will often die from the shock of fluctuating temperatures, snapping off from wind or burning in direct sunlight.

“I’ve been growing approximately 2,000 seedlings indoors and have been lugging them in and out of the house every day for over a week now,” Garland said. “They’re beginning to toughen up, but the evening temps are too chilly to risk leaving them outdoors overnight.”

It is possible to purchase seedlings from nurseries that have already been hardened off, Garland said.

“It’s always a good idea to ask the nursery staff whether things are hardened off and ready to plant before making any assumptions,” she said. “Your local nurseries want you to be successful, so they’ll be a terrific resource.”

It’s understandable that given warm weather, some gardeners jumped the gun and have already planted seedlings. Even though temperatures are going to drop this week, Garland said there are things they can do to protect the plants.

The easiest thing to do is cover the plants at night with plastic or fabric. Those covers can either be removed when daytime temperature rises, or have ventilation to allow daytime warmer air to circulate.

“The key is making sure those cozy spaces don’t get too hot during the day,” Garland said. “If it’s just a few plants, you may want to consider putting them back in their pots and giving them a little more transition time.”

Before placing any seedlings into the ground, gardeners should check the forecast for nighttime temperatures for the next week to 10 days.

“We have had late frosts in the past,” Garland said. “It could easily happen again.”

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Julia Bayly

Julia Bayly is a reporter at the Bangor Daily News with a regular bi-weekly column. Julia has been a freelance travel writer/photographer since 2000.