When to Transplant Seedlings Outdoors

Weather forecasts vary by region, so instead of setting firm dates, gardeners often use the average date of the last frost as a benchmark to determine when to start their seeds indoors and also when to transplant them outdoors. Depending on the species and variety, transplanting is generally done a few weeks before or after the last frost. Some plants need a bit more warmth, so you may wait for the air to warm up before moving them outside. In southern Canada and Northern United States, last frosts occur at the end of April or May, while farther North it can stretch to early June, even late June in some places! Some areas, especially those at higher elevations, can experience frosts later into the spring. You can easily find the specific the average date of the last frost for your area with a simple web search.  

Frost-Free Date

However, the last frost date is just an average: 50% of the time, the last frost will occur before this date, and the other 50%, it will follow. It also varies depending on the period used to calculate the average, such as the last five years or the last twenty years. I prefer using the frost-free date rather than an average, especially considering current climate disturbances. It’s as simple as adding two weeks to the average date of the last frost in your area. So, if you are advised to transplant your tomatoes 2 weeks after the last frost, add another 2 weeks to the average date, for a total of 4.  

Temperature

These dates are useful for estimating the transplant date and, by extension, the date for sowing seeds indoors or outdoors. However, they should be understood as an approximation rather than a strict guideline. Actual conditions can vary significantly from year to year, especially considering climate change impacts. As the crucial time approaches, the weather offers no guarantees. Forget these dates and rely on the temperature to make a final decision. Since plants have both above ground and underground parts, it is advisable to check both air and soil temperatures. There are thermometers specifically designed to measure soil temperature, but personally, I just use a cooking thermometer (though apparently this is not recommended!).  

Cool Weather Vegetables

Cool weather vegetables, such as beets, carrots, lettuces, onions, and parsley, can be transplanted as soon as the soil temperature reaches 7°C. These crops tolerate cooler conditions well and can be planted early in the season. As for air temperature, they can be transplanted when nighttime temperatures also stay above 7°C without any risk of frost. Other cool weather vegetables, like turnips, various types of cabbages (broccoli, kale, cauliflower), as well as Swiss chard and corn, prefer slightly warmer soil, ideally around 12°C, but they can withstand cooler air temperatures.  

Tomatoes

For tomatoes, which are more sensitive to cold, it is crucial to ensure that the soil temperature is at least 15°C before transplanting. Moreover, to avoid thermal shock, it is best if nighttime temperatures do not drop below 12°C.

Warm Weather Vegetables

Warm weather vegetables, such as beans, peppers, cucumbers, squashes, and melons, require even warmer conditions. Wait until the soil temperature reaches at least 21°C before sowing or transplanting. These plants also benefit from higher nighttime air temperatures, between 18 to 21°C, to optimize their growth and prevent developmental slowdowns.   The temperatures for transplanting various vegetables are generally correct, but there can be regional adaptations and variances. For example, the exact soil temperature that is ideal for a specific vegetable might differ slightly based on local conditions or the variety of the plant.  

Acclimatization

 It’s easy to see why some gardeners have seeding calendars or spreadsheets to sort out all these dates! Additionally, it’s necessary to plan for acclimating our plants to outdoor conditions like sunlight, wind, and rain, a week or two before transplanting them into the garden. To simplify, place your seedlings outside in a shaded, wind-protected area as soon as the temperature exceeds 15°C, or even 21°C for warm climate plants. Every 2-3 days, move them so they get more and more light, taking care to avoid midday full sun until the very end. Your seedlings can stay outside overnight, but as soon as the temperature drops below 15°C, bring them back indoors. If they need to stay inside for more than 48 hours, the acclimatization cycle must start over.