Timing is everything with tomatoes in Montana. Start them indoors too early and you end up with leggy, rootbound plants weeks before it is safe to set them out. Start them too late and they never have time to ripen in our short season. Getting the start date right is one of the most important things you can do for a good tomato harvest in Missoula.
The short answer is that most Missoula gardeners should start tomato seeds indoors somewhere between late March and the middle of April. Here is how to figure out the right date for your garden and how to give those seedlings a strong start.
Count Backward From Your Transplant Date
The way to nail down your start date is to work backward from when you will actually plant the tomatoes outside. Tomatoes are a warm-season crop and cannot handle frost, so they go out after our average last spring frost around May 19, and only once the soil has warmed up. For most gardens that means setting tomatoes out in late May or the first few days of June.
Tomato seedlings need about six to eight weeks indoors before they are ready to transplant. For the full month-by-month breakdown, see our seed starting schedule. Count back six to eight weeks from your planned transplant date and that is when to sow your seeds. Counting back from late May to early June lands you in late March through mid-April. If you tend to wait for warmer soil before planting out, lean toward the later end. If you use season extension like row covers or water-filled protectors to plant earlier, you can start a bit sooner.
Give Seeds Warmth to Germinate
Tomato seeds germinate best in warm soil, ideally in the 75 to 85 degree range. That is warmer than most houses, so a seedling heat mat under the trays makes a big difference, speeding up and evening out germination. Once the seeds have sprouted, you can take them off the heat.
Keep the seed-starting mix lightly moist but not soggy while you wait for germination, which usually takes about a week in warm conditions. A humidity dome or loose cover over the tray helps hold moisture until the seedlings break the surface, at which point you remove it so air can circulate.
Strong Light Is Not Optional
The most common mistake with indoor tomatoes is not enough light. A windowsill, even a sunny south-facing one, rarely gives seedlings enough light in our late-winter weather, and they respond by stretching tall and floppy reaching for it. Those leggy seedlings make weak transplants.
A simple grow light, or even an ordinary shop light with bright bulbs, kept just a couple of inches above the seedlings and run for fourteen to sixteen hours a day, produces stocky, strong plants. Raise the light as the seedlings grow to keep it close. This one change does more for transplant quality than almost anything else.
Pot Up and Harden Off
As the seedlings grow, they will outgrow their original cells. Moving them into larger pots, called potting up, gives the roots room to develop and keeps the plants growing strong rather than getting rootbound. Tomatoes can be planted deeper each time you pot up, since they grow roots along their buried stems, which makes for a sturdier plant.
Before the tomatoes go outside, they need to be hardened off. This means introducing them to outdoor conditions gradually over a week or two, starting with an hour or two in a sheltered, shady spot and working up to full sun and longer days. Our spring sun and wind can shock tender indoor-grown plants fast, and MSU Extension recommends hardening warm-season transplants for two to three weeks before planting out. Skipping this step is a quick way to set back or kill plants you spent two months growing.
Get the timing and the care right, and you will be setting out healthy, stocky tomato plants right when the weather is ready for them, which is the foundation of a good harvest in our short season. For help choosing the right types, see our guide to the best tomato varieties for Montana’s short season. If you want help planning your seed-starting calendar and tomato strategy for Missoula, that is what I do. Take a look at the vegetable garden consulting page and reach out.