Determining Planting Dates

Average time to read: 4.0 minutes

Getting the timing right when starting seeds is an important part of your success in the garden. Seeds are living things and they know when conditions are right for growing. If you sow seeds into the wrong conditions, your results probably won’t be great. So how do you get the timing right?

This type of garden instruction is challenging because the “right” thing to do can vary drastically between regions and from year-to-year. Rather than getting caught up in nitty gritty details, our goal here is to give you a foundation that you can use across the board. A strong understanding of the most important factors will serve you well when starting out. In time, you’ll develop your “gardeners intuition” and begin to feel more confident.

To put it as simply as possible: start seeds when the following 3 conditions can be met:

1. The soil has warmed to the right temperature

2. There is no danger of frost damage to the plant

3. The season is at least as long as the plant’s lifecycle

Let’s unpack that.

1. Start seeds when… the soil has warmed to the right temperature

For every variety, there is an optimal temperature for germination and a different optimal temperature (usually a range) for growth. Many new gardeners do not realize that these numbers refer to soil temperature, not air. Soil is almost always a few degrees cooler than air. Especially during the shoulder seasons (ie spring and fall) when overnight lows can dip below freezing even though daytime highs are quite warm.

Seeds that are direct-sown outdoors must be sown into soil that is warm enough to trigger germination. Your seed packets might indicate a specific temperature. This is common for corn and beans which are particularly fussy and want 16C soil. However, most packets will not be that specific.

Seed packets of varieties that will germinate in cooler soil may instruct you to sow sometime during the spring. This is common for radish, lettuce, and peas. Squash and melons, however, need warm soil to germinate and don’t transplant well. So those packets are more likely to instruct you to direct-sow in June or early-summer.

In any case, this is where you apply your gardener’s intuition. It is impossible for seed packet instructions to be broadly applicable to gardeners across Canada. And they certainly don’t know when you’re having unseasonable weather. So for example, if you have a packet of seeds that instructs you to direct-sow in late spring but you have 3 feet of snow on the ground and a blizzard warning in effect, wait! And conversely, you can decide to risk sowing ahead of schedule if your garden is really well sheltered and the 2-week forecast looks perfect. There is a reason packets are a little vague. They’re counting on your gardener’s intuition and so should you.

2. Start seeds when… there is no danger of frost damage to the plant.

Even after soils warm up and direct-sowing has begun, frosts are to be expected. All plants have varying degrees of frost tolerance, from a lot to none at all. No matter what, young seedlings will be at risk from frost in the spring. As gardeners, it’s our job to keep an eye on the local weather forecast and provide frost protection whenever there is a frost warning in effect.

Depending on their frost tolerance, some plants will require more protection than others in the event of a frost. Here are the basics:

Frost Hardy: Plants that will survive consecutive overnight frosts

Asparagus, broccoli, Brussels sprouts, cabbages, lettuce, onions, radishes, rhubarb, peas, spinach, parsnips, early potatoes

Semi-frost Hardy: Plants that will tolerate overnight frost once in a while

Beets, carrots, cauliflower, celery, late potatoes, early sweet corn

Semi-frost Tender: Plants that will tolerate near-freezing temperatures for a short time (ie single night) but may show minimal damage below 4C and significant damage below 0C.

Snap beans, sweet corn, tomatoes

Frost Tender: Plants that will not tolerate frost. Crop failure below 4C.

Lima beans, cucumbers, eggplant, muskmelons, peppers, pumpkins, squash, watermelons

3. Start seeds when… the season is at least as long as the plant’s lifecycle

This is what usually determines whether a variety has to be started indoors early or direct-sown outdoors. Long-season varieties (like onions, leeks, eggplants, tomatoes, and peppers) have a long lifecycle. Thanks to our Canadian climate, it is common to start them indoors early. They still have the soil temperature requirements we went over in the first point. However, you’ll be simulating those conditions indoors. More on that here.

Starting plants indoors early allows us to fake a longer growing season, giving your long-season plants the time they need to mature and bear fruit. However, this 3rd point is also relevant to direct-sowing late in the growing season. Many short-season varieties can be sown throughout the season but be careful not to sow too late. There needs to be at least enough time left in the growing season for the final sowing’s crops to mature.

Now you’re ready!

When it comes down to it, you can use the seed packets as a guide. You can use your provincial government’s official planting dates. Or you can keep it as simple as possible and wait until your garden soil is at least 10C before planting anything into it.

Above all else, don’t worry about being perfect; you can’t be! Mother Nature is your boss in the garden. Even when you do everything “right,” something may still go wrong. That’s just part of gardening.

Want more like this? We’ve designed a garden planning guide to help you keep track of your seed starting, transplanting, and harvest dates. Check it out here.

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