Check before you worry
First, find out whether there really are none. Tubers form on underground stems close to the seed piece, about 4–6 inches down, and around flowering they are often marble- to egg-sized. Reach in gently with your hand at the edge of one plant, or lift a single plant with a garden fork, rather than digging up the row. Small tubers mean the plant is on schedule. Not every variety flowers, and flowering isn't a sign that tubers are finished; it tends to come around the time new potatoes are first worth digging.
It may be too early
Timing explains most surprises. University of Maryland Extension notes that small new potatoes can be dug 6–8 weeks after planting, usually once plants have flowered, and that tubers keep growing until the plants are killed by frost, insects, disease or age. Most varieties need about 90–120 days to mature. For storage potatoes, Maryland suggests leaving tubers in the ground for 2 weeks after the vines die so the skins thicken. If your plants are blooming and green, you're likely in the middle of tuber growth, not the end.
Heat and nitrogen
Heat is the next suspect. Tubers form best when the soil is about 60–70°F, and University of Illinois Extension notes they fail to form when soil reaches 80°F. Potatoes planted late, grown in dark containers in full sun, or growing in bare soil through a hot early summer can make big tops and few tubers. Straw after hilling keeps soil cooler, about 10°F cooler according to Illinois, and more evenly moist. Too much nitrogen pushes vines at the expense of tubers; UC advises avoiding excess nitrogen because it causes excessive vine growth.
Water and hilling
Water matters most while tubers are forming, and Maryland notes that too much or too little water is linked to many tuber problems, including cracked and misshapen tubers. Long dry spells at that stage can leave few potatoes. Hilling matters too. Mounding loose soil around the stems as plants grow keeps the forming tubers covered and shielded from sun, which turns them green. Hoe carefully near the plants, since Illinois warns that developing tubers are quick to get cut.
What to do now
Keep the soil evenly moist through bloom and after, and stop feeding nitrogen. Mulch with straw once the hills are formed. Check under one plant again in 2–3 weeks to see how the tubers are sizing up. If the vines are healthy, give them time. If they were stripped by Colorado potato beetles or disease, the tubers stop growing, so deal with the pests first. Once vines die back on their own, wait 2 weeks and dig on a dry day.
Next season
Next year, plant certified seed potatoes once the soil reaches about 45°F, often 2–4 weeks before your last spring frost, so tubers form before summer heat. Maryland suggests fertilizing before planting and about 6 weeks later, when tubers begin to form, rather than loading the bed with nitrogen. In hot regions, choose early varieties and mulch heavily. Write down your planting date so you know when to start checking.
Growing it again? When to plant potato for your ZIP code
- How do you grow potatoes in a container or grow bag?Use an opaque 10–15 gallon container with drainage holes, filled with potting mix and compost. Plant 2–3 certified seed potato pieces and keep adding mix as the stems grow.
- Why are my potatoes green, and are they safe to eat?Potatoes turn green when light reaches them. The green comes with natural toxins called glycoalkaloids. Peel slightly green potatoes deeply. Throw out ones that are heavily green or taste bitter.
- Can you give a plant too much nitrogen, and what does it look like?Yes — excess nitrogen produces very dark green, lush leafy growth at the expense of flowers and fruit, and in high concentrations can burn roots and cause wilting.
- Why are some of my potatoes hollow inside when I cut them?Hollow heart in potatoes is caused by rapid, uneven growth — often after a period of drought followed by heavy watering or rain — and is not a disease.