Introduction

Planting vegetable seeds or transplants at the correct time is important for a successful garden. The correct timing is determined by two factors: the soil temperature required for seeds to germinate and the temperature tolerance of the plants.

The best planting dates can vary from season to season. Planting dates also differ from one location to another based on the microclimatic effects of urban areas, natural terrain, moisture, sunlight, wind exposure, and garden devices such as cloches and mulches.

Because of this variation, we can only suggest a range of safe planting dates based on statistical dates of the last frost in the spring (Figure 1) and the first frost in the fall (Figure 2). You should note the current weather conditions and projections as well as your site’s conditions to determine the correct planting date for a specific crop and variety.

Making several plantings within these date ranges can increase the likelihood of success. Several plantings will also extend the harvest season over a longer period. If you make only one planting, then make it about midway through the range.

Table 1 indicates spring planting dates, while Table 2 suggests appropriate dates for fall garden planting.

Author

Rosie Lerner
Department of Horticulture and Landscape Architecture

soil temperature

Many summer vegetable crops do not grow well until soil temperatures are warm. In years when cool air temperatures and rainfall do not allow the soil to warm up, delay planting warm season crops such as beans, tomatoes, squash, sweet potatoes, and sweet corn until the soil temperature has reached at least 60ºF for optimum germination and/or growth. Otherwise, seed and root rot disease and related disorders are likely.

For more experienced gardeners, soil temperature measurements offer an alternative to planting according to frost dates. Soil can be warmed early by using black or clear plastic mulches. Seeds that require warm temperatures for germination can then be planted successfully before the recommended dates. However, protection against late frosts for the seedlings may be required. Table 3 provides seed germination temperatures for several vegetables.

When deciding fall planting dates, be sure to consider the days to maturity for your crop. If planting after September 1, expect a longer time to maturity the later you plant. Choose faster maturing cultivars when possible. Also note that soil can be too dry in late summer for optimum germination. Be prepared to irrigate frequently until crop is established.

A heatmap of Indiana with individual counties outlined. It shows frost dates in spring. Figure 1. Frost Dates in Spring. There is a 10% chance that the last occurrence of 32ºF will occur after these dates. About two weeks earlier, that chance increases to 50%. Tender crops may need some protection from frosts.
A heatmap of Indiana with individual counties outlined. It shows frost dates in winter. Figure 2. Frost Dates in Fall. There is a 10% chance that the first occurrence of 32ºF will occur before these dates. About 2 weeks later, that chance increases to 50%. Tender crops may need some protection from frosts.

Table 1. Some common vegetables grouped according to their relative requirements for cool and warm weather.

Cool season crops for early-spring planting

Hardy (tolerates hard frost) plant 4-6 weeks before last spring frost (see Figure 1)

  • asparagus4
  • broccoli2
  • Brussels sprouts2
  • cabbage2
  • collards3
  • horseradish4
  •  kale3
  • kohlrabi
  • leek2
  • mustard3
  • onion3, 4
  • peas1
  • potato4
  • radish1
  • rhubarb4
  • spinach3
  • turnip1

Semi-hardy (tolerates light frost) plant 2-4 weeks before last spring frost

  • beet3
  • carrot1
  • cauliflower2
  • celery2
  • chard3
  • chinese cabbage2
  • lettuce3
  • mustard3
  • parsnip1

Warm season crops for later-spring or early-summer planting

Tender (damaged by frost) plant after average last spring frost (minimum air temp 50 F)

  • bean¹
  • tomato²
  • sweet corn¹

Very Tender (damaged by light frost and air temperature below 50 F) plant at least two weeks after average last spring frost (minimum air temp 60-65 F)

  • cucumber³
  • eggplant²
  •  melons²
  •  okra²
  • pepper²
  • sweet potato⁴
  • pumpkin³
  • squash³

Table 2. Cool season plants for late-summer or fall planting (plant at least 4-8* weeks before first fall frost) (See Figure 2)

  • beet3
  • broccoli2
  • Brussels sprouts2 (transplant by mid-summer)
  • cabbage2
  • collards2
  • green onion2
  • kale2
  • kohlrabi2
  • lettuce3 
  • mustard greens1
  • radish1
  • spinach3
  • turnip2
  • mustard greens1

* When deciding fall planting dates, consider the days to maturity for your crop and choose faster maturing cultivars when possible.

¹ typically direct-seeded

² typically transplanted

³ can be either direct-seeded or transplanted

⁴ typically vegetatively propagated

Table 3. Soil Temperatures for Vegetable Seed Germination

Table 3. Soil Temperatures for Vegetable Seed Germination
Vegetable Optimum/Optimum Range (°F) Minimum/Maximum (°F)
Bean 80/60-85 60/95
Bean, Lima 85/65-85 60/85
Beet 85/50-85 40/95
Cabbage 85/45-95 40/100
Carrot 80/45-85 40/95
Cauliflower 80/45-85 40/100
Celery 70/60-70 40/85
Chard, Swiss 85/50-85 40/95
Cucumber 95/60-95 60/105
Eggplant 85/75-90 60/95
Lettuce 75/40-80 35/85
Muskmelon 90/75-95 60/100
Okra 95/70-95 60/105
Onion 75/50-95 35/95
Parsley 75/50-85 40/90
Parsnip 65/50-70 35/85
Pea 75/40-75 40/85
Pepper 85/65-95 60/95
Pumpkin 95/70-90 60/100
Radish 85/45-90 40/95
Spinach 70/45-75 35/85
Squash 95/70-95 60/100
Tomato 85/60-85 50/95
Turnip 85/60-105 40/105
Watermelon 95/70-95 60/105