When should I plant tomatoes outside?
Short answer: Transplant tomatoes outdoors after all danger of frost has passed and the soil is warmer than 60°F (Utah State Extension). Tomatoes grow best with nights of 60-70°F (Clemson), and flowers drop when nights fall below 50°F. Start seeds indoors 6-8 weeks before your last frost date, and harden seedlings off for about a week first.
When is it safe to plant tomatoes outside?
Tomatoes are warm-season plants, and frost can kill them. Extension services use two checks before transplanting:
- Frost: Wait until all danger of frost has passed. Find your average date in our last frost date guide.
- Soil: Utah State University Extension says to transplant when soils are warmer than 60°F.
The University of Minnesota Extension gives the same rule in plain words: plant out after frost danger is past “and the soil has warmed.”
An average last frost date is not a guarantee. Frost can still arrive after it, so watch the forecast for the week after planting.
What temperatures do tomatoes need?
| Condition | Figure | Source |
|---|---|---|
| Soil at transplanting | Warmer than 60°F | Utah State |
| Best daytime air | 70-80°F | Clemson |
| Best nighttime air | 60-70°F | Clemson |
| Flowers drop | Nights below 50°F or days above 95°F | Utah State |
| Fruit aborts | Above 90°F (e.g., under row covers) | Utah State |
So frost-free is the minimum, not the ideal. If nights are still dipping below 50°F, expect some dropped flowers until they warm up.
Use a soil thermometer at planting depth in the morning, when soil is coolest. Plant once it reads above 60°F.
When should I start tomato seeds indoors?
| Source | Recommendation |
|---|---|
| Clemson | Sow indoors 6-8 weeks before your last frost date |
| Utah State | Allow 6-8 weeks to grow transplants |
| U of Minnesota | Start 5-6 weeks before planting outside (mid-April for most of Minnesota) |
To find your start date, count back from your planned transplant date. For example, a May 15 transplant date minus 6 weeks (42 days) is April 3. Minus 8 weeks (56 days) is March 20.
Seed-starting temperatures from the sources:
- Germination: Utah State germinates seed at 80°F. Minnesota suggests a heat mat holding the flat at 75-85°F until seedlings emerge.
- After sprouting: Utah State grows seedlings at 65-70°F. Minnesota says 70°F soil is ideal.
- Depth: Minnesota recommends sowing seeds 1/4 inch deep.
- Light: Clemson warns that low light produces tall, spindly transplants.
Utah State says a ready transplant has 5-7 leaves and a well-developed root system.
How do I harden off tomato seedlings?
Indoor seedlings need a gradual move outside. Clemson says to begin about a week before transplanting.
- Put plants outside with wind protection and a couple of hours of sun (Minnesota).
- Increase sun exposure over the next week or two (Minnesota).
- Reduce watering, but do not let plants wilt (Clemson).
- Bring them in if night temperatures approach freezing (Minnesota).
Clemson notes that leaves may develop purplish veins during hardening. That is part of the process.
What is the best way to transplant tomatoes?
Transplant in the evening or on an overcast day (Clemson), or in late afternoon or on a cool, cloudy, calm day (Minnesota). This reduces wilting.
Utah State adds a mulch timing note. Wait to apply organic mulch until soil is warmer than 75°F.
Can I plant tomatoes before the last frost?
Only with protection. Utah State says plastic mulch and row covers let plants go out “several weeks before the last frost date.” Options include hotcaps, plastic tunnels, and fabric covers.
Covers can overheat plants on sunny days. Utah State says to ventilate them when air temperatures exceed 80°F. Use a thermometer under the cover to check.
If a late frost is forecast after planting, see how to protect plants from frost.
Quick answers
Is 50 degrees too cold for tomato plants? Nights below 50°F can cause tomatoes to drop flowers, according to Utah State Extension. Clemson lists 60-70°F as the best night range.
Can I plant tomatoes in April? It depends on your last frost date and soil temperature, not the month. Plant once frost danger has passed and soil is above 60°F.
How many weeks before the last frost should I start tomato seeds? Clemson says 6-8 weeks before your last frost date. Minnesota suggests 5-6 weeks before planting outside.
What soil temperature do tomatoes need? Utah State says to transplant when soil is warmer than 60°F. For germinating seeds indoors, it uses 80°F.
How long should I harden off tomato plants? Start about a week before transplanting, per Clemson. Minnesota suggests increasing sun exposure over one to two weeks.