Transplant Threshold
Wait until soil reaches 60 degrees at 4-inch depth and nights consistently exceed 50 before planting outdoors.
Heirloom tomatoes are frost-tender and halt growth below 50 degrees. Use a soil thermometer—metal-stemmed and accurate—to check at 4-inch depth mid-morning, mid-week. Multiple tests across your garden spot help, as sun exposure and soil type affect readings. Raised beds warm 3 to 5 days sooner, so plant earlier there; low-lying or shaded beds may need another week.
Nighttime lows matter as much as daytime warmth. If nights dip below 50 after transplanting, use row covers or cloches. Avoid planting during cold snaps—even if soil is warm, sudden cool air stuns roots and delays establishment. Water thoroughly after planting to settle soil around roots, but hold off on fertilizing for two weeks to prevent burn and encourage root spread.
