What to do now
Mildest step first.
- Dig up a few seeds: soft, mushy seeds mean rot; hollowed-out seeds or small white maggots mean seedcorn maggot.
- Check the soil 2 inches down for a few days in a row, and wait for about 60°F for beans, squash and melons and at least 50°F for corn (65°F for supersweet types) before replanting.
- Replant shallowly into soil that drains, and hold off if a cold rain is forecast within the next day.
- If manure or a cover crop was turned in recently, wait 2 to 3 weeks before replanting large seeds.
Cold, wet soil
The fungi and water molds that rot seeds, such as Pythium, Rhizoctonia and Fusarium, are present in virtually all soils, UC notes, and seeds planted in cold soil saturated with water are the most vulnerable. Seeds can rot before they germinate, or seedlings can decay before they come up. University of Maryland Extension notes that corn and bean seed in particular are likely to rot in cold, wet soil, and Mississippi State notes that Pythium seed decay is common and that cool, wet conditions especially favor it. If you dig and find seeds that are swollen, soft or mushy, this is the likely cause.
Why cold soil matters
Cold soil gives those organisms time. Oregon State's table of soil temperatures and days to emergence shows how slowly seeds come up when it's cool. Corn takes about 22 days at 50°F but 7 at 68°F. Beans barely germinate at 50°F and take 16 days at 59°F, against 8 at 77°F. Cucumbers barely germinate at 50°F and take 13 days at 59°F. Peas are tougher but still take 36 days at 41°F and 13 at 50°F. Each extra day in cold, wet soil is another day for rot to start.
Crops most at risk
Large-seeded are the usual victims. Oregon State lists 60°F as the minimum soil temperature for beans, cucumbers, squash and melons, and 50°F for corn. Some sources set the bar higher for the touchiest crops. Clemson suggests waiting for 55°F to reduce most seedling rots in sweet corn, and 65°F for extra-sweet varieties; the University of Minnesota notes that cold-soil emergence problems below 65°F are most common in supersweet . Oregon State's lima bean page notes that soil below 60°F gives poor stands. The Pacific Northwest disease handbook notes that wrinkled-seeded garden peas are more prone to rot than smooth-seeded field peas, and suggests sowing them, when practical, once the soil 2 inches down is above 55°F.
Chilling injury
Beans, limas and corn can also be injured by cold water itself, in the first hours after sowing. Cornell notes that bean seed is sensitive to chilling while it first takes up water, that the first 24 hours after planting are the most critical, and that very dry or low-vigor seed is the most sensitive. Penn State describes the same risk for corn in the first 24 to 48 hours, when cells in the seed can rupture, and Nebraska notes that injured seed leaks sugars and other cell contents. Iowa State, writing about soybeans, suggests not planting when a cold rain is forecast within 24 hours. Cornell and Oregon State suggest letting bean and lima seed take up moisture slowly from humid air rather than a soak. Pre-sprouting indoors before planting in cool soil is a separate option that Maryland suggests where it suits the crop.
Hollowed-out seeds
If the seeds are hollowed out rather than mushy, look for seedcorn maggots, the small white larvae of a fly. Maryland notes they are most common on large-seeded warm-season crops such as corn, beans, peas and melons, and that the smell of decaying manure and organic fertilizer attracts the flies. Iowa State notes they are worst in cool, wet springs when plants grow slowly, and that freshly tilled ground draws egg-laying females. Iowa State suggests waiting about two weeks after tilling or spreading manure before planting. Maryland suggests turning under at least 3 weeks before planting, waiting until the soil is warm, and planting as shallowly as possible so the seeds come up quickly.
Other causes
A few other things make rot more likely. Old or poorly stored seed has less vigor, Maryland notes, and the PNW handbook suggests planting vigorous seed. Seeds planted too deep may not come up; Maryland's rule is 2 to 3 times the seed's diameter, and it notes that soil that crusts when dry can keep seedlings from breaking through. Watering a cool bed heavily after sowing keeps it wet longer, and UC IPM and Mississippi State both warn against overwatering.
Replanting
The fix is usually to wait and replant. UC IPM suggests waiting for the soil and air to warm, using raised beds and not overwatering, and Maryland suggests raised beds wherever soil stays wet for days after rain. The PNW handbook suggests replanting in dry, warm weather if a stand is lost. Measure the soil where the seed will sit: Wisconsin suggests checking 2 inches down at the same time of day for three days in a row, and Clemson checks 4 inches down for corn. To warm a bed sooner for warm-season crops, Colorado State notes that black plastic can be laid early and removed before planting. Fungicide-treated seed of corn, beans and peas is another option that Mississippi State and Clemson mention.
Read the whole label first: it must list your crop, and its directions (amount, timing, gear, bees, days before harvest) are the law. If this page and the label differ, follow the label.
- Green BeanA warm-season legume that keeps coming as long as you keep picking.
- Lima BeanA warm-season legume that asks for patience and punishes impatience.
- CornA wind-pollinated grass that needs a block of plants, a block of nitrogen, and a block of your garden.
- PeaA cool-season crop that rewards early sowing and quits when summer arrives.
- CucumberA thirsty vine that wants warm soil, steady water, and something to climb.
- Damping OffSeedlings collapse at soil level with a pinched, rotted stem — happens quickly in wet, cool conditions.
- Gray Mold (Botrytis)Gray-brown fuzzy mold on fruit, flowers, or stems — soft, collapsing tissue beneath the coating in cool, wet conditions.
- Crown RotThe base of the plant turns brown and soft at the soil line, and the plant collapses — caused by wet-soil pathogens attacking the crown.
- Root RotPlants wilting despite wet soil, with brown or slimy roots — wet-soil pathogens destroying the root system.
- What soil temperature do I need to plant vegetables?It depends on the crop. Lettuce, spinach and onion seeds can sprout in soil as cold as 35°F. Peas, carrots, beets, radishes, cabbage and kale need about 40°F, potatoes 45°F, and corn and tomatoes 50°F. Beans, cucumbers, peppers, squash and melons need 60°F. Seeds come up faster and more evenly when the soil is well above these minimums.
- 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.
- Are old seeds still good?Often, yes. Most vegetable seed stays good for 2 to 5 years if it's kept cool and dry, but onion, parsnip and parsley fade after about a year. The way to know for sure is a germination test: 10 seeds in a damp paper towel for 7 to 10 days.
- 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.