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Gardening Columns

Planting by the Calendar

First published on on February 12, 2009, by Suzanne DeJohn

Should I start my sunflowers indoors? Why are my petunia seedlings so small? My tomato plants are already huge and I can't plant them outside for weeks, what should I do?

Experienced gardeners sometimes forget all the questions they had when they first began to garden, and starting seeds can be especially baffling. Which plants do you start as seeds indoors and when? Which should you sow directly in the garden?

Unless your gardening expertise was gathered at your parents' or grandparents' side, you probably learned what you know by a combination of research and trial and error. In the spirit of sharing my trials and minimizing your errors, here's how I decide what to sow when:

Make a Planting Calendar
There's plenty of how-to information on the mechanics of starting seeds -- choosing soil, watering, light, etc. -- but less on the timing aspect. One reason is because the timing is based on where you live and, more specifically, on your average last frost date.

The easiest way to determine your last frost date is to ask a gardening neighbor. Or call your Master Gardeners or Extension Service, or search online maps. Because it's based on averages, you may get a hard frost days or even weeks later, but it's a place to start. Always be prepared to protect tender plants for a few weeks after this date has passed.

For example, the average last frost date for Cambridge, Vermont is around the end of May; tradition here calls for planting out your tender transplants on Memorial Day weekend. However, because the holiday falls a bit earlier than usual this year, I'm marking the following Saturday, May 30, as my last frost date. (I'll be sure to have plenty of old sheets and row covers at the ready should a late frost threaten.)

The next step is researching each crop's preferences. To determine if and when I should start seeds indoors, I divide seeds into these categories:

Short-season cool-weather plants: Fast-maturing vegetables that prefer cool temperatures are usually best sown directly into the garden, rather than started indoors. They germinate in cool soil, grow quickly, and are ready to harvest by early summer. Examples include spinach, lettuce, radishes, beets, and peas. Because they can tolerate a light frost, they can be sown outdoors in the garden a few weeks before the last frost date. Sweet peas, bachelor's buttons, and larkspur are cool-weather annual flowers that should be direct-sown in early spring, too.

Short-season warm-weather plants: Plants that like warm weather and mature quickly are also best sown directly into the garden. Beans, corn, and sunflowers are good examples. These plants grow quickly and don't like having their roots disturbed by transplanting. Wait until after the last frost date, and make sure the soil has warmed and dried out from spring rains before planting the seeds or they're likely to rot.

Long-season warm-weather plants: Many favorite garden crops, such as tomatoes, require a long, frost-free growing season. If you plant tomatoes from seed after the last spring frost, you'll be lucky to get a tomato or two before the first fall frost. That's why tomatoes, peppers, and eggplants are set in the garden as transplants (seedlings that have been started indoors or in a greenhouse). You can start the seeds yourself or purchase the transplants. This category also includes petunias, ageratum, and impatiens.

Long-season cool-weather plants: Some cool-season crops also do better when set in the garden as transplants. Broccoli, cauliflower, and cabbage suffer in the heat of summer, so setting transplants out early -- a few weeks before the last spring frost -- gives them time to mature before hot weather arrives. Snapdragons and pansies fall into this category, too.

Marigolds, zinnias, cosmos, squash, and pumpkins can go either way; you can direct sow or start them indoors.

Now, back to our calendar.

Calculating Planting Dates
Planting times can be a little confusing at first, but a calendar simplifies things. For example, you should start broccoli seeds indoors about six weeks before transplanting. And you can transplant them into the garden two weeks before the last frost date. Starting from my last frost date of May 30, I count back two weeks to May 16 and make a note there: "transplant broccoli." Now I count back six weeks and mark my broccoli seed-starting date.

Then I do the same for each type of seed. You should start pepper seeds about eight weeks before transplanting time, and because they don't like cool weather, it's best to wait until a week after the last frost to set them out. I make a note on June 6: "transplant peppers" and count back eight weeks to April 25 as the day to start my pepper seeds.

Although it may be tempting to get a jump on the season by planting seeds earlier, unless you have a full-fledged greenhouse it's probably not worth it. Compact, sturdy plants will transplant more readily and actually grow faster in the garden than gangly, oversized, potbound plants.

Other Considerations
Some plants take such a long time to reach transplant size that I don't bother growing them myself. Impatiens, pansies, begonias, geraniums, and petunias fall into this category. They can take up to three months to reach the size you see for sale in stores. Tomatoes, peppers, eggplant, squash, cosmos, and zinnias, on the other hand, take a more manageable six to eight weeks.

I could sow my zinnia, cosmos, and squash seeds right in the garden, but I prefer to start them indoors so they reach the blooming stage earlier, and because I find it easier to weed around sturdy transplants than tiny seedlings.

Harden off indoor-grown seedlings to acclimate them to outdoor conditions. Place them outdoors for increasing amounts of time over the course of a week or so, starting with an hour in light shade and working up to leaving them out in full sun for the whole day. Plants that haven't been hardened off frequently suffer from sunburn and stunted growth; don't skip this step.

With the erratic weather New England invariably experiences in spring, don't depend on Mother Nature to tell you when to plant seeds and transplant seedlings outdoors. Although you can always try setting out a few plants early, wait to plant your main crop until your calendar says it's time. It's heartbreaking to see rows of tomato and basil seedlings -- seedlings that you've nurtured for months -- blackened by frost. Better safe than sorry.


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