When to Plant Vegetable Seeds and Transplants Outdoors

December 11, 2024|The Rusted Garden|

The mistake many gardeners make is focusing on the ambient or surrounding air temperature. While that does influence growth, soil temperature is the key to higher rates of seed germination and vigorous transplant growth. Placing a large tomato plant in cold soil, doesn’t lead to bigger plants and production earlier in the season.  Plants will actually sit and chill while they wait for warming roots to signal rapid growth.  Seeds will just sit in cold soil increasing their chances of mold and fungal decay.

Understanding the general soil temperature ranges for your vegetables, when direct seeding or planting transplants, will improve germination, early growth, and overall harvest yields. You don’t need to actually measure the soil temperatures or be perfectly accurate with the temperature values. Knowing when the general temperature ranges come to your garden, increases success.

Rising soil warmth influences and triggers vigorous plant root and shoot growth.  Soil temperature is generally measured at 4 in/10 cm deep in soil that is not mulched or covered. Warmth is relative to the preference of the crops you are planting.

‘General’ Guidelines by Soil Temperature

  • Below 40°F (4°C): Most seeds will not germinate.
  • 40–50°F (4–10°C): Some cool-season crops like peas, spinach, and lettuce can germinate.
  • 50–60°F (10–16°C): Perfect for seeding many cool-season vegetables like broccoli, carrots, and beets. Cool-season transplants do better at the higher end of the range.
  • 60–70°F (16–21°C): A general range for transplants/seeding for both cool and warm-season vegetables.
  • 70–85°F (21–29°C): Best for transplants/seeding warm-season crops like tomatoes, peppers, and cucumbers. Crops preferring cool soil tend to flower and produce seeds (bolt).
  • Above 85°F (29°C): Germination can become sporadic for many crops, and high soil temperatures can stress plants. The process of bolting definitely begins for any remaining cool-season crops.
  • Above 95°F (25°C): Gemination may not occur and plants often drop flower or shut down fruit formation and production.
The Rusted Garden

Cool-Weather Crops

Optimal soil temperature for cool-weather crops is 65° F/18.3° C. These crops can germinate at lower temperatures like 50° F/10° C, and in some cases lower. However, germination speed and transplant growth increase significantly as soil temperature approaches the optimal range. You can really start direct seeding crops that enjoy cool soil when the soil is around 55°F/12.7° C. This holds true for planting transplants. Every garden varies and attempting to plant at optimal soil temperatures, limits growing time for the crops.

Very often we start cool-weather seeds indoors because they germinate more quickly (5-10 days) in a home temperature of 65° F/18.3° C. They can be started indoors when outdoor soil temperature sits too cold and the ground may even be frozen. Seeds would survive and can be planted outdoors, but gemination may take 4 times longer (20-40 days) in the much colder soil. Seeds that sit in cold damp soil have a greater chance of rotting.

Here are general soil temperature ranges for direct seeding some cool-weather crops. I find transplant do best when soil temperatures around 55° F/12.7° C.

  • Spinach: 45–75°F (7–24°C)
  • Lettuce: 40–75°F (4–24°C)
  • Kale: 45–75°F (7–24°C)
  • Carrots: 45–85°F (7–29°C)
  • Radishes: 45–85°F (7–29°C)
  • Beets: 50–85°F (10–29°C)
  • Peas: 40–75°F (4–24°C)
  • Broccoli: 50–85°F (10–29°C)
  • Cauliflower: 50–85°F (10–29°C)
Gary Pilarchik The Rusted Garden

Warm-Weather Crops

Optimal soil temperature for warm weather crops is around 80° F/27.7° C.  This is the temperature in which seeds germinate and establish quickly and transplants establish and grow the best. Roots growing in this temperature thrive, and accelerate overall plant growth. This does not mean seeds and transplants won’t grow when soil temperatures are lower. They do at a significantly lower rate when soil temperatures fall below 60° F/ 15.5° C. However, that leaves a range where adequate growth can occur below the optimal temperature of 80° F/27.7° C.

The best way to use this information is to target soil temperatures around 65° F/18.3° C for direct sowing and for transplants, understanding the soil warmth is moving nicely toward the optimal soil temperature.  You don’t have to wait for the optimal temperature to plant. It should be trending nicely in that direction. Warm rains are also a good signal soil temperature is moving upwards.

Warm crops prefer warmer soil temperatures, usually 60–85° F or 15.5–29°C to germinate, be transplanted, and grow well. Germination can be impacted once soil temperatures exceed 85° F/29.4° C. The key is to think ‘general ranges’.

  • Tomatoes: 60–85°F (16–29°C)
  • Peppers: 65–85°F (18–29°C)
  • Cucumbers: 60–95°F (16–35°C)
  • Squash (Summer and Winter): 60–95°F (16–35°C)
  • Eggplant: 70–90°F (21–32°C)
  • Beans: 60–85°F (16–29°C)
  • Corn: 60–95°F (16–35°C)
  • Melons (Watermelon, Cantaloupe): 70–95°F (21–35°C)

My next blog article will focus on when to start seeds indoors.

Good Luck in Your Gardens,

Gary – Please Follow Me on YouTube: The Rusted Garden