A tomato set into cold ground in an Idaho garden sits there and sulks, while one planted a little later catches up fast. The pattern holds every year: the plants go out in late spring once frost has passed and the soil is warm, and the seedlings start indoors well ahead of that.
When Is the Best Time to Plant Tomatoes in Idaho?
Plan on planting tomatoes outside in May, with June as a fallback when spring runs cold. The calendar follows the weather, since frost kills the plants and cold soil keeps them from growing.
- In March and April, seeds are started indoors, so the transplants are sturdy by the time the garden is ready for them.
- In May, transplants go into the garden once all danger of frost has passed and the soil has warmed to about 60°F.
- In late May, planting is safest when temperatures stay consistently above 60°F, because tomatoes stall when nights turn cold.
- In June, planting can still be done if a late cold snap or wet ground held you back in May.
- Through summer, the plants keep growing, though in very hot spells large-fruited types may stop setting fruit until it cools.
At the Boise reference point, a low valley, the last spring frost falls between April 23 and May 7, and the first fall frost arrives between October 7 and October 21. Most of Idaho is higher and colder, with far shorter seasons, and the state spans USDA hardiness zones 3b to 7b (a system built on the average yearly minimum temperature). Gardeners in the mountain valleys should lean toward the later end of the planting window.
Idaho — the year and the numbers
- State
- Idaho
- USDA zones
- 3b-7b
- Last spring frost
- April 23 to May 7
- First fall frost
- October 7 to October 21
- Annual rain
- 12 in
- Lawn zone
- transition-season
- Source
- NOAA NCEI 1991-2020 Climate Normals, 32F freeze probabilities · Boise
When to Start Tomato Seeds for Idaho
Seeds go into pots indoors in March and April, from mid-March to early April. That gives the seedlings six to eight weeks to grow into good transplants before they head outside.
Starting earlier tempts many gardeners, but leggy, crowded seedlings do worse than younger ones, and they need plenty of light to stay stocky. The page on starting tomato seeds indoors in Idaho covers the details of trays, light and soil.
How to Plant Tomatoes in Idaho
Planting goes smoothly when each step waits for the weather to cooperate. These gestures follow the order you will use them in.
- Before planting day, harden the seedlings off outdoors for one to two weeks, so tender leaves toughen up in sun and wind instead of scorching.
- When the soil has warmed to about 60°F and frost is behind you, move the plants into the garden, because roots stay idle in cold ground.
- At planting, set each transplant deep, burying part of the stem, so more roots form along it and the plant stands firmer.
- Right after planting, put the stake or cage in place while the roots are still small, so you do not damage them later.
- After setting the plant, water it in well so soil settles around the roots and air pockets close.
- Once the soil is warm, spread mulch around the plants to hold moisture and keep the roots from swinging between hot and cool.
Mulching too early traps the chill in the ground, so wait until the soil has really warmed.
When to Seed, Plant and Harvest Tomatoes in Idaho
The year runs in a simple order: seeds indoors in early spring, transplants outdoors in late spring, then picking until frost ends the season.
Timing at the Boise reference point is looser than in the high country, where nights stay cool longer and the frost-free season is shorter.
| Task | Best timing in Idaho | Why then |
|---|---|---|
| Start seeds indoors | March to April, mid-March to early April | Gives seedlings six to eight weeks to grow before planting out |
| Plant transplants outside | May, and June if needed | Frost is past and the soil is near 60°F |
| Plant in late spring | Late May, when temperatures hold above 60°F | Warm nights keep plants growing instead of stalling |
Harvest has no set month, so watch the weather. Pick ripe fruit as it colors, and when a frost warning comes in the fall, bring in the mature green tomatoes before the plants are killed.
Where to Get Tomato Advice in Idaho
Local help is easy to reach, and it matters most when a soil test or a sick plant is the question. Your yard may differ from the valley floor, especially if you garden at higher elevation.
- The University of Idaho Extension publishes growing guidance suited to the state’s climate, and it is a good first stop before planting season.
- Your county extension office can arrange a soil analysis in spring, and it can help identify a leaf problem if you bring a sample of the affected plant.
Common Questions
Can You Plant Tomatoes in Idaho in June?
Yes. June works when spring stayed cold or wet and May planting slipped. Transplants set out then still have time to grow and bear, as long as frost is behind you and the ground is warm.
What Tomato Varieties Do Well in Idaho?
Early Girl is a popular choice for early production with mid-size fruit. Whatever you choose, hybrids with VFN resistance are recommended. For short seasons, early to early mid-season types that ripen their fruit in 60 to 70 days suit cooler gardens.
Should You Grow Tomatoes in Pots in Idaho?
Pots work well, especially smaller varieties, which often stay compact and spread only a short distance around. A container also warms faster in spring, which helps in cooler parts of the state.