Seedlings planted by Noami Brautigam start to sprout March 16 at Dickey Hill Farm in Monroe. Credit: Gabor Degre | BDN

Gardening season in Maine won’t begin for a month or so, but that doesn’t mean you can’t start preparing for a fertile crop.

Maine’s northerly climate means that the growing season starts a little later than many other places throughout the country — and ends a little sooner. The key for a successful growing season here is being prepared.

Starting seeds indoors is a great way to extend the growing season, while helping you get your gardening fix until you can get your hands in the soil. Some crops that are sensitive to cold or have especially long growing seasons that stretch beyond the frost-free window in Maine are only possible to grow if started indoors.

The date at which freezing conditions are projected to be no more is called the last frost date. To find your last frost date, check the National Gardening Association website which provides a range of dates and the probability of freezing or below freezing temperatures for each based on data from the National Oceanic and Atmospheric Administration. Dave’s Garden’s search tool also provides last frost projections using data from the National Climatic Data Center.

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The last frost dates will vary slightly depending on where you live in Maine. In Bangor, for example, the last frost is projected to be between May 18 and May 30, while in Portland it is predicted to be between May 2 and May 15. Meanwhile, in Presque Isle, the last frost is projected to be anywhere between May 21 and June 3.

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Here are some popular crops that you should start indoors — and when you should start them — until spring has sprung. (Note, though, that while general tips are helpful, be sure to check the seed packets for specific recommendations based on the variety you are growing.)

Tomatoes

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Perhaps the most popular home garden crop, tomatoes are sensitive to cold and have long growing seasons, so they should be started indoors. The University of Maine Cooperative Extension recommends starting tomatoes six to eight weeks before transplanting the seedlings, depending on the variety. Tomatoes should generally be transplanted at least a week or two after the danger of the last frost has passed — so, if you have tomato seeds you want to plant this year, now is the time to start.

Peppers

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According to the University of Maine Cooperative Extension, peppers should be started seven to ten weeks before the danger of the last frost has passed — perhaps even longer for hot peppers, which have longer growing seasons and are especially sensitive to cold temperatures (heat mats are especially helpful for trays of hot pepper seedlings). Peppers seedlings should be transplanted two to three weeks after the threat of frost has passed, after the soil has had a chance to warm up.

Eggplant

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Eggplant has a long growing season, so in colder climates like Maine, it is best to start them indoors. The University of Maine Cooperative Extension says that eggplant seedlings should be started indoors seven to ten weeks before transplanting them outside. Transplant eggplant seedlings two or three weeks after the last frost has passed, so the soil has time to warm above the optimal temperature of 70 degrees Fahrenheit. You can even use an instant-read thermometer, like the one you use for cooking, to make sure your soil is warm enough for your seedlings.

Broccoli & cauliflower

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Start broccoli and cauliflower seeds six to eight weeks before transplanting them into the garden. The seedlings should be transplanted when they have at least two sets of true leaves — not the tiny little green springs that pop up on seedlings, but the beautiful, veined leafy things. Because the plants are fairly cold hardy, transplanting broccoli and cauliflower can be done about two weeks before the last frost date.

Cabbage

Cabbage seedlings should be started indoors six to eight weeks before transplanting outside. Once the plants have developed two to three true leaves, they are ready for transplant. Like broccoli and cauliflower, they can be transplanted a few weeks before the last frost (fun fact: all these plants are all in the brassica family, along with Brussel sprouts and kale) in order to avoid the cabbage-loving pests that pop out once the weather warms.

Head lettuce

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Leaf lettuce — arugula, spinach and the like — can be directly sown even before the last frost has passed. Leafy lettuces will grow quickly, even in indoor conditions. Head lettuce, however, is a different beast. Head lettuce like iceberg, romaine, butterhead and summercrisp should be started indoors four to six weeks before transplanting the seedlings outside right when the danger of frost has passed. They can handle a little bit of chill, so no need to be as cautious with these as you are with, say, tomatoes and peppers.

Onions

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Onion seeds should be started indoors nine to ten weeks before transplanting the seedlings outside. Onion seedlings will tolerate a light frost, so they can be transplanted up to two weeks before the last expected frost date. The Maine Organic Farmers and Gardeners Association recommends starting certain onion seeds in February or March. That is to say, if you want to have onions this year in your garden, get a move on starting those seedlings now if you haven’t already.

Celery

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Celery seedlings should be started 12 to 15 weeks before transplanting them in the ground, once they have four to five true leaves. In the spring, celery seedlings are best transplanted two to three weeks before the last frost. If you haven’t started celery seeds already, it might be best to wait for a fall planting. Celery seedlings should be sown 10 to 12 weeks before the first frost because the crop is sensitive to warm temperatures. In Maine, the first frost generally happens at some point between mid-September and early October, depending on where you live.

For a fall celery planting, you can start seedlings at some point in the next few weeks. Don’t get discouraged if your celery seedlings don’t pop up at the same time as the rest of your seedlings, though. The University of Maine Cooperative Extension says that celery can take 9 to 21 days to germinate.

Remember, seedlings started indoors need to be hardened off by bringing them outside during the day for about a week before transplanting them into the ground. This allows seedlings to build defenses to the elements before they are moved permanently outside. Take the hardening off period into consideration when planning. Be sure to also use proper technique when transplanting your precious, tender seedlings into their new homes in order to minimize the shock of being moved to your garden bed.