When to Plant Tomatoes in North Idaho

Seed Planting

• Plant tomato seeds indoors about 10 weeks before you plan to set the seedlings into the outdoor garden soil. The average date of last frost in northern Idaho varies depending on elevation. At lower elevations, the average date of last hard frost is generally around the middle of May, but at higher elevations that date extends into early June. Check the average date of last frost in your specific area and plant the seeds accordingly.

Hardening Off

• Introduce the tomato plants to the outdoor temperatures and environmental conditions before planting them in the garden. This is called hardening off and prevents the plants from being stressed when going from indoors to the outside garden. Harden off the tomato plants by setting them outside for several hours during the day. Increase the outdoor time each day for seven to 10 days until the seedlings are outside 24 hours a day for at least three days straight. Protect the seedlings if the temperature drops to below 50 degrees F to prevent damage.

Cold Frame

• A cold frame allows you to plant tomato seedlings into the garden early. The cold frame protects the plants from freezing temperatures by acting as a removable greenhouse. Set up the cold frame about two weeks before the planting date to warm the soil. You can start planting the seedlings into the cold frame when the soil has warmed to at least 60 degrees F. Harden off the seedlings by opening the frame for several hours a day. Gradually increase the open time for seven to 10 days, using the same time frame as for hardening off seedlings set outside. The cold frame can be removed once there is no longer a risk of freezing temperature.

Transplanting

• Transplant tomato seedlings directly into an outside garden once there is no longer a risk of a hard frost and the air temperatures stay above 50 degrees F. The actual date varies each year, based on weather conditions.

Considerations

• Tomato plants are sensitive to frost and are damaged easily when the outdoor temperature drops below 40 degrees F. Cover outdoor-growing seedlings with sheets or lightweight blankets when there is an expected frost. Place stakes around the tomato seedlings and drape the covering over the stakes to keep frost off the plants. Remove the covering promptly in the morning once the sun comes out and the temperature warms above 40 degrees F.