When to Plant Seeds in Your Garden
There's something about the ground thawing and temperatures warming that makes us eager to plant seeds in our gardens. However, with lots of frosty nights still in the short-term forecast, it's probably wise to hold off on planting most seeds for now.
There are several types of vegetables that can be seeded when the ground is cool. Just because it's possible doesn't mean it's a good idea!
Even though they can germinate in cool soil, they will germinate much faster as the soil warms up. Therefore, it's best to wait for a warm spell, and get your seeds in the ground on the first warm day of that warm spell. That way, they'll pop up faster with less risk of rotting in cold, wet soil - and will generally out-perform plants that struggled to germinate in cool soils.
The amazing thing about cold-hardy plants is that once they're up, they can handle some cold conditions! They'll be sizing up long before frost-sensitive plants can be set in the garden, providing early-season harvests. They will benefit a lot from some simple protection from the elements (such as frosty nights and heavy winds), which is covered in this post on how to extend your gardening season.
Explanation of Common Terms
"As soon as the soil can be worked": this usually means that planting can begin when the soil is fully thawed, loose, and not too wet. This is an ideal stage for planting young fruit trees, and for certain vegetable crops.
"Once the ground has warmed": a lot of seeds will do best when planted into warm soil. How warm is warm enough? It depends on the crop! The list below gives some guidance on when to plant crops in our area (southern Ontario / zone 5). You'll note that certain crops must be direct-seeded at the end of May or early June. These crops MUST be sown into warm soil, or they will struggle to germinate.
IDEAL DIRECT-SEEDING DATES FOR COMMON VEGETABLES
As with any crop, the seeding dates are flexible and dependent on the actual weather at the time. For example, if a crop can be planted in early April, go ahead with it IF the ground is relatively dry and workable, and the temperatures are above freezing at night.
On the other hand, if it's still below freezing at night and the ground is cold and wet... then it's best to postpone the seeding for a week or two.There are also several cold-hardy crops that can be transplanted as seedlings in the early spring, which is detailed in this post.
Crops to plant every week (for a continuous supply): arugula
Crops to plant every 2 - 3 weeks (which is also the amount you'll be eating every 2 - 3 weeks): beans, beets, sweet corn, carrots, peas, radishes, spinach (~ once / month), cilantro, dill, salad turnips (these are different than rutabaga; these salad turnips are varieties such as Hakurei, Niseko, Japanese salad turnips, etc)
Crops to plant in greater volume in mid-July for winter storage: beets, carrots, winter radishes
Crops to plant densely (~1 seed / cm in the rows): baby lettuce, baby kale, arugula
A note about spinach: it loves cool weather, and may bolt prematurely in the summer unless it's given shade and adequate moisture; definitely plan for a late-summer planting to enjoy through the winter (with simple protection from the elements)
Transplanting for Earlier Harvests
Any crop marked with an asterick (*) in the chart can also be effectively transplanted. Check out this post for more info on when to transplant. This shortens the span of time that they're in the garden between planting and harvest. In other words, you can get a harvest a lot faster from transplanted seedlings than from direct-seeding in the soil.