Gardening in Maine: zones, frost dates and when to plant

Coastal, central and northern Maine

University of Maine Cooperative Extension’s Planting Chart for the Home Vegetable Garden sets its dates for central Maine. It gives two dates for each crop, the earliest and latest safe planting. From there the rule is simple. On the coast, plant 10 to 14 days earlier. In northern Maine, plant 10 to 14 days later. That puts coastal and northern gardens three to four weeks apart, the biggest regional difference in Maine vegetable gardening.

Start with the frost dates for your city above, then use the chart’s offset for your part of the state.

The short season is the main limit

UMaine Extension’s Bulletin #2752, Extending the Gardening Season, puts it directly. Most vegetables are sensitive to cold and grow only in the frost-free months, which keeps Maine’s growing season short. Everything else follows from that: which varieties you choose, when you transplant and how you protect plants at each end of the season.

Choose varieties with the season in mind. Bulletin #2190, Vegetable Varieties for Maine Gardens, advises northern Maine gardeners to grow only early varieties of warm-season crops such as tomatoes, peppers and melons, so they ripen before frost. Farther south, early and midseason varieties both ripen in most years. Some of the bulletin’s picks:

• Tomatoes: extra-early Early Cascade, New Girl and First Lady; early First Pik, Royal Mountie and Sunstart. For small fruit, Sun Gold, Matt’s Wild, Juliet and Sweet Million.
• Sweet corn: early yellow Kandy Kwik and Seneca Horizon; bicolor Seneca Arrowhead and Fleet.
• Peppers: thin-walled sweet Ace, New Ace and Giant Marconi; thick-walled Red Knight, Boynton Bell and King Arthur.

The bulletin also recommends checking seed catalogs for disease resistance. For tomatoes and peppers, look for resistance to Fusarium, Verticillium and tobacco mosaic virus.

Planting in sequence

UMaine’s “Keep Your Garden Growing” guide spreads planting across four stages:

1. Early spring, as soon as the ground can be worked: broccoli and cabbage transplants, plus seeds of endive, lettuce, peas, radish, spinach, turnip and onion sets. Beans, corn and early potatoes follow about a week later.
2. Midspring, about two weeks before the average last frost: cauliflower transplants and seeds of beets, carrots, parsnips and Swiss chard.
3. Early summer, when soil and sun are warm: tomatoes, peppers and eggplant, plus cucumbers, melons, squash, pumpkins and winter potatoes.
4. Late June into early August: another round of beets, carrots, lettuce and radish, with broccoli, cabbage and cauliflower set out in early August. Transplants go in later than seed because hot weather slows their growth.

Don’t let a warm spell in late April fool you. UMaine Extension says head lettuce, fennel, chicory, onions, shallots and scallions can go in then. Most other seedlings should wait, and all need hardening off before transplanting. If you plant early anyway, mulch and use floating row covers or cold frames on cold nights.

Stretching the season

Bulletin #2752 covers the main tools Maine gardeners use to add weeks at either end of the season:

• Black plastic mulch warms the soil less than clear plastic but suppresses weeds. Press it flat against the soil so it warms well. Clear plastic belongs only on fallow ground for solarizing, where it can heat soil to as high as 125°F. Around crops it can cause heat injury.
• Row covers raise daytime air temperature around plants by up to 10°F and hold residual heat overnight. Secure the edges, and remove the covers when crops need pollinating.
• Cold frames protect from frost better than floating row covers and are useful for hardening off seedlings. Open them on sunny days so they don’t overheat.
• Hoop houses and hot caps add protection for earlier harvests. Hoop houses cost more and take more work.

Acid soil and soil testing

UMaine Extension notes that Maine soils tend to be acid, and soil that is too acid limits how well many plants grow. Bulletin #2286, Testing Your Soil, explains how to find out what yours needs. Test kits are available from your county UMaine Extension office or from the Analytical Lab and Maine Soil Testing Service at the University of Maine. Sample in early spring once the frost is out of the ground, or in fall before the ground freezes. Results take about two to three weeks and include lime and fertilizer recommendations. Home garden samples are also checked for lead. Test at least every three years.