Our previous blog discussed some of the basics of seed starting and why it's such a valuable skill to have as a gardener. In that blog, we discussed the importance of having a quality heat mat and thermostat to regulate the temperature of the soil in your seed starting cups or trays. Having the optimal soil temperature in your seed starting trays will ensure you get the best germination rates and that your seeds germinate in a timely manner.
Although some related vegetables (tomatoes, peppers, eggplant, for example) have similar optimal germination temperatures, the optimal temperature for seed germination can vary from one crop to the next. Warm-season vegetables prefer warmer soils for germination, while cool-season vegetables will germinate in slightly cooler soils.
We typically don't have to use heat mats when germinating cool-season crops because the outside temperatures are warm in late summer when we're starting these seeds for fall planting. In the case of those cool-season vegetables, we just need to make sure that the soil isn't too warm. Some lettuce varieties, for example, won't germinate well in warmer soils.
Warm-season vegetables like tomatoes and peppers are typically started in late winter or early spring when outdoor temperatures are still cool. As a result, we need to provide heat via a germination mat to get those soil temperatures warm enough for high germination rates.
GERMINATING SEEDS INDOORS OR OUTDOORS
Optimal germination temperatures not only apply to seeds that are started indoors or in a greenhouse, but it's also very important for seeds that we plant directly in our garden soil. While they can be transplanted, vegetables like corn, beans, and peas will do just fine when direct-seeded into your garden. You just need to make sure that your soil is warm enough for seed germination.
Below we've compiled a list of the most common vegetables grown in backyard gardens. For each of those vegetables, we've listed a germination temperature range and an optimal germination temperature. The range provides the lowest and the highest temperature at which those seeds will germinate.
It's important to note that germination may be very slow or minimal at the bounds of these ranges. That's why it's important to try and have your soil (in a tray or in the ground) as close to the optimal germination temperature as possible when planting these seeds.
| CROP |
GERM |
OPTIMAL |
| BEANS | 65°F - 85°F | 80°F |
| BEETS | 50°F - 85°F | 85°F |
| BROCCOLI | 50°F - 85°F | 75°F |
| BRUSSELS SPROUTS | 50°F - 85°F | 75°F |
| CABBAGE | 50°F - 85°F | 75°F |
| CARROTS | 50°F - 85°F | 75°F |
| CAULIFLOWER | 45°F - 85°F | 80°F |
| COLLARDS | 45°F - 85°F | 75°F |
| CORN | 50°F - 90°F | 85°F |
| CUCUMBERS | 60°F - 90°F | 85°F |
| EGGPLANT | 60°F - 95°F | 85°F |
| KALE | 45°F - 85°F | 65°F |
| KOHLRABI | 45°F - 85°F | 80°F |
| LEEKS | 45°F - 95°F | 75°F |
| LETTUCE | 40°F - 85°F | 75°F |
| OKREE | 60°F - 90°F | 85°F |
| ONIONS | 45°F - 90°F | 75°F |
| PARSNIPS | 50°F - 85°F | 70°F |
| PEAS (ENGLISH) | 40°F - 85°F | 75°F |
| PEAS (FIELD) | 65°F - 95°F | 85°F |
| PEPPERS | 65°F - 95°F | 85°F |
| PEANUTS | 65°F - 85°F | 80°F |
| PUMPKINS | 70°F - 95°F | 85°F |
| RADISH | 55°F - 85°F | 65°F |
| RUTABAGA | 45°F - 85°F | 70°F |
| SPINACH | 40°F - 75°F | 65°F |
| SUMMER SQUASH | 60°F - 95°F | 85°F |
| SWISS CHARD | 40°F - 95°F | 80°F |
| TOMATOES | 60°F - 95°F | 85°F |
| TURNIPS | 45°F - 85°F | 65°F |
| WATERMELON | 65°F - 95°F | 90°F |
PRINT AND POST IT!
We hope this table of germination temperatures is useful and that it helps you better develop your planting schedule for spring or fall gardening. Feel free to print this table and post it in your garden shed for quick reference. If you can get your seed starting trays close to these optimal temperatures, you should have great germination rates. And for the direct-seeded crops on this list, wait until temperatures warm in spring (or cool in fall) so that you're planting in garden soil that's close to the optimal temperature listed above.
