Idaho · zones 5b–7b
When to plant tomatoes in Idaho.
Sow or transplant between May 18 to Jul 3 in a typical year, keyed to Idaho's median last frost around May 11. Dates below are state medians; your ZIP code shifts them by days to weeks.
The 2026 windows
Tomatoes dates for a typical Idaho season.
1 · Start indoors
Apr 6 to May 22
About 6 weeks before set-out. Warm spot, real light.
2 · Set out
May 18 to Jul 3
Keyed to the ~May 11 last frost. Sow 0.25" deep, 24" apart.
Harvest
~73 days later
~12 lb per plant over ~8 weeks.
Idaho spans more than one climate: Zone 6 is the most common zone, but the state runs 5b to 7b. That's why the planner works from your ZIP code's frost dates, not your zone: mountain and coastal gardens in the same state can be three weeks apart.
Idaho growing notes
A full-size slicing tomato needs 73 of Idaho's 140 frost-free days from transplant, and the slack left over is thinner than it sounds.
The math behind the slicer
Idaho's frost-free run stretches about 140 days start to finish, and a slicing tomato here takes roughly 73 of them, counted from transplant, to load fruit worth calling a slicer rather than a green softball. That leaves a couple of months of slack on paper, but a transplant set into ground that's still cold sits without growing instead of racing toward its 73-day mark, and that stall comes straight out of the cushion. Treasure Valley gardens, lower and warmer, spend that month finishing fruit. Gardens in the state's colder 5b pockets often spend a chunk of it just waiting for the soil to catch up first.
Cool nights are the other variable, and the useful version of this is a behavior worth watching for, not a number worth memorizing. A plant blooming hard but setting no fruit while nights are still running cold is not sick and doesn't need feeding; it's waiting on warmth. Set resumes on its own once nights hold higher for a run of days, and nothing sprayed or fed onto the plant speeds that up. Track which spot in the yard stalls out this way: give the earliest transplant the warmest wall or the bed that holds heat overnight, and leave the coldest corner for a crop that shrugs off the chill.
Blight follows the hose more than the sky
Idaho's air runs dry enough that early blight, late blight and septoria leaf spot don't spread here the way they do somewhere humid, where the air alone keeps leaves damp long enough for spores to move. Here, the wet-leaf conditions that get all three going are usually something the grower supplied: an evening sprinkler, or plants set close enough at 24 inches that air stops moving through the row. Switching to drip or a soaker line at the soil surface removes the one variable a gardener actually controls, and for a lot of Idaho tomato patches, that single change is most of the disease control the crop needs.
When one of the three shows up anyway, septoria usually leads: small dark spots with pale centers, low on the plant, working upward. Early blight follows with target-ringed spots on the older leaves. Late blight is the one to act on the same day it's found, spreading fast and turning whole leaflets a greasy brown-black within days, usually after a run of wet weather. Pull and bag any leaf with that look rather than composting it, since composting just restarts the problem nearby. A plant that loses its lower canopy to blight by late summer, deep into the season's cushion, still has time to finish fruit already set, not much time to start more.
The visible pests
Tomato hornworms are the pest every new gardener panics over and every experienced one manages by not panicking, since a caterpillar the exact green of the leaf it's eating is hard to spot directly. Look for the evidence instead: dark pellet droppings on leaves or on the ground under a stripped branch. Before doing anything about one found, check its back for small white rice-grain shapes; those are a parasitic wasp's cocoons, and a hornworm carrying them is best left exactly where it is. It's already dying and about to release more wasps into the garden, which is pest control this crop gets for free.
Aphids are the steadier nuisance, clustering on new growth and the underside of leaves where a casual glance misses them. A firm water spray knocks back a light infestation before it's worth reaching for anything stronger. At 24 inches between plants and something like 12 pounds of fruit per plant across a season, a run of six or eight slicers covers most households' sauce and slicing needs with room to spare, which makes losing one plant to pests or a blight patch an inconvenience rather than the whole crop.
In Idaho: watch for
The pests that find tomatoes here.
The short season here, about 140 frost-free days, keeps tomato, slicing in front of tomato hornworm only briefly. One early row cover usually carries a planting through the worst of it.
Big green caterpillars strip leaves and leave black droppings.
Organic fix: Handpick at dusk; spray Bt on the young ones.
Clusters on new growth, sticky leaves, curling tips.
Organic fix: Blast with water, then insecticidal soap or neem.
Dark spots with concentric rings, starting on the lowest, oldest leaves.
Organic fix: Remove affected leaves, mulch to stop soil splash, space for airflow.
Greasy, water-soaked blotches in cool, wet spells. No rings, and not bottom-up.
Organic fix: Pull infected plants fast, keep leaves dry, use neem as a preventive.
Septoria leaf spot
Many small tan spots with dark brown edges, low leaves first, no rings inside.
Organic fix: Strip the lowest leaves, mulch against soil splash, stake for airflow.
Your season math: Idaho gives tomatoes about 140 frost-free days, and it needs roughly 73 days to a first harvest. It's a one-planting crop, so time that single sowing well.
Planting tomatoes in Idaho: questions
When should I plant tomatoes in Idaho?+
In a typical Idaho year, sow tomatoes or set out transplants between May 18 to Jul 3, keyed to the state's median last frost around May 11. Your exact dates shift with your ZIP code and microclimate.
When do I start tomatoes seeds indoors in Idaho?+
Start seeds inside about 6 weeks before set-out: roughly Apr 6 to May 22 for a typical Idaho season.
What USDA zone is Idaho?+
Idaho spans zones 5b–7b, mostly zone 6. But frost dates, not zones, set planting time. The free planner uses your ZIP code's actual frost dates.
How many tomatoes plants do I need?+
Depends who you're feeding. The crop page has exact counts per household size, or the free planner does the math for your family in about 60 seconds.