When to Plant Potatoes

The rule: soil at 50°F, 2 to 3 weeks before the last frost

Potatoes don’t go by the calendar. They respond to soil temperature. The working threshold from Texas A&M, Utah State and the University of Maine is about 50°F. Texas A&M measures it 4 inches down, where the seed piece sits. In an average year, soil reaches that temperature around 2 to 3 weeks before the last spring frost. Utah State gives 14 to 21 days, and Texas A&M about 3 weeks. Use the dates for your zone above as your starting point, then confirm with a soil thermometer.

Why not plant earlier?

Cold, wet soil is the danger. Maine notes that seed pieces may sprout a little sooner in colder soil, but if they sit cold and wet for long, they rot. A rotten seed piece has nothing left to regrow from if the shoots get frozen. Texas A&M warns that plantings made too early can have their tops frozen off by a spring frost. The sources disagree on how hardy the leaves are. Clemson says plants can take a light frost, while Texas A&M says the tops can’t take frost at all. Either way, there is little to gain from planting ahead of warm soil.

Why not plant later?

The crop has to make its tubers before summer heat. According to Utah State, tuber set and filling are suppressed once soil goes above about 90°F. Small, odd-shaped tubers are the typical result. Clemson notes that potatoes keep growing longer and yield more while the soil stays cool. Each week you delay in spring is a week less of good tuber weather at the other end. Most varieties need roughly 85 to 120 days from planting to a mature crop, so count back from when your summers turn hot.

Getting a head start in cold regions

In the northern tier and New England, the soil is usually what holds planting back, not the air. Two techniques help:

• Black plastic: Utah State suggests planting through black plastic mulch, which warms the soil and lets you plant and harvest earlier.
• Green sprouting: Maine recommends spreading seed potatoes in a bright spot at about 70°F with high humidity before planting. They grow short, sturdy sprouts that emerge quickly and set tubers early.

Maine also points out that lighter, well-drained soils warm and dry out sooner than heavy ground. If you have a choice of beds, use the sandiest one for your earliest potatoes.

Fall planting: Texas yes, much of the Southeast no

A second crop works where autumn is long and mild. Texas A&M recommends planting about 110 days before the first expected fall frost. Use small, uncut seed potatoes about 1½ inches across, because cut pieces rot in hot soil. Fresh seed potatoes are dormant, so keep them warm and damp in the shade (under moist burlap or mulch) for about 2 weeks before planting until small sprouts show.

Not every southern state agrees. Clemson does not recommend fall potatoes anywhere in South Carolina because pest and disease pressure is higher then. University of Arizona’s calendar for the Phoenix-area low desert also shows no fall planting.

Warm-winter areas

Where frost is rare, as in Florida, along the Gulf Coast and in the low desert (roughly zones 9-11), potatoes are a fall-to-winter crop dug before summer heat. UF/IFAS gives different windows for North, Central and South Florida, getting earlier as you go south. University of Arizona places low-desert planting in winter. The dates for your zone above reflect these regional windows.

Quick checklist before planting

• Soil 4 inches deep at about 50°F
• Bed drains well and isn’t soggy
• Certified seed potatoes on hand, cut and healed if you’re using pieces