What to do now
Mildest step first.
- Push a soil thermometer 4 inches into the soil in the morning and check it for several days in a row.
- Sow lettuce, spinach and peas once the soil holds at 40°F or warmer.
- Wait for 50°F to sow corn, and 60°F to sow beans, cucumbers and squash or to set out tomato and pepper plants.
Soil temperature by crop
These are the minimum soil temperatures for seeds to sprout, with the range where they sprout best in parentheses. 35°F: lettuce (40–80°F), spinach (45–75°F) and onion (50–95°F). 40°F: peas (40–75°F), carrots (45–85°F), beets (50–85°F), radishes (45–90°F), and cabbage and kale (45–95°F). 50°F: corn (60–95°F) and tomato seed (60–85°F). 60°F: beans (60–85°F), cucumbers (60–95°F), peppers (65–95°F), squash and watermelon (70–95°F), and pumpkins (70–90°F). The numbers come from a germination table by J.F. Harrington of UC Davis, which Oregon State and several other extension services publish. Potatoes are planted as pieces of tuber, not seed, and Colorado State lists 45°F as the soil temperature to plant them.
Minimum versus best
The minimum is the coldest soil in which seed will still sprout, not the best time to sow. Near the minimum, seeds come up slowly and unevenly, and seed that sits in cold, wet soil is more likely to rot. Within the best range they come up quickly and together. In practice, sow once the soil is reliably at their minimum, and give a few extra degrees.
Setting out transplants
Plants grown indoors follow their own rule. The University of Maryland and Utah State both say to set out tomatoes once the soil is at least 60°F, and Maryland's planting chart asks for the same 60°F soil, with nights above 50°F, for all summer crops. Maryland also says peppers, eggplant and melons do better with soil closer to 70°F. The planting check on this site uses 60°F for tomatoes and peppers.
Where sources differ
Extension sources do not always agree. For beans, Maryland says at least 60°F and preferably 65°F, while Colorado State lists 55°F measured at 6 inches deep. Harrington's table gives 60°F for cucumbers, but Clemson says to plant them when the soil reaches 70°F, and Maryland suggests 70°F before sowing squash and pumpkins. For corn, Clemson says at least 50°F, but to wait for 65°F for extra-sweet varieties, so check your seed packet.
How to measure
Colorado State suggests measuring at 8 a.m., 4 inches deep, when the soil is near its coldest for the day (6 inches deep for beans). One warm afternoon does not mean the soil has warmed, so look for several mornings in a row at or above the number you need. Each vegetable's plant page on this site has a "Can I plant this week?" check that shows this week's soil temperature and night lows for your area, so you can compare them with these numbers.
- LettuceA cool-season leaf crop that thrives in spring and fall, sulks in summer heat.
- SpinachA cold-weather green that gives you leaves when almost nothing else grows.
- PeaA cool-season crop that rewards early sowing and quits when summer arrives.
- CarrotA root crop that rewards patience and deep, rock-free soil.
- BeetA two-for-one crop — sweet roots below, peppery greens above.
- Why aren't my seeds germinating?Cold soil is the most common culprit — most vegetable seeds stall below 60°F, and old or improperly stored seed may not be viable at all.
- What is a last frost date and how do I actually use it?Your last frost date is the average date of the final freezing night in spring — it's a probability, not a promise, and smart planting adds a buffer of 1–2 weeks beyond it for cold-sensitive crops.
- Why do my transplants look stunted and aren't growing?Cold soil, root-bound roots, or fertilizer burn can all stall a transplant — check soil temperature first before diagnosing anything else.
- How early can I plant potatoes?Plant seed potatoes about 2 to 4 weeks before your last frost date, once the soil is at least 45°F and dry enough to work. Potato leaves are hurt by frost, but if early shoots are frozen, the plant usually sends up new ones within two weeks.
- How do I plant asparagus crowns?In early spring, as soon as the soil can be worked, dig a trench about 6 to 8 inches deep and set the crowns 12 to 18 inches apart with the buds facing up. Cover them with 2 inches of soil and fill the trench as the spears grow. Don't harvest the first year; most extension services say to wait until the third spring for a real harvest.