When to Harvest Hydroponic Potatoes

Reading the plant

The leaves tell you where the tubers are in their growth. Tubers usually start forming around the time the plant flowers, then spend the following weeks filling out. As the plant reaches the end of its season, the lower leaves yellow first, then the whole plant yellows, flops and dies back. That die-back is the sign that the tubers have finished growing.

• Flowering: tubers are starting. Too early to harvest, but a good time to switch to a lower-nitrogen nutrient.
• A few weeks after flowering: small new potatoes are ready to take, often around seven to eight weeks after planting.
• Foliage yellowing and dying back: the tubers have stopped growing.
• A week or two after die-back: the skins have had time to set, and the crop is ready to store.

Days to maturity give a rough guide too: early varieties usually finish in about 70 to 90 days, mid-season varieties in 90 to 110, and late varieties in 110 days or more. Cool weather and weaker light stretch those numbers.

Taking new potatoes

New potatoes are young tubers with thin, tender skins. To take a few without ending the crop, slide your fingers gently into the medium around the edge of the basket, feel for tubers about the size of an egg or smaller, and twist them free. Leave the smaller ones and the plant's main stem alone, then smooth the medium back over so no light reaches the rest. New potatoes don't store well, so eat them within a few days.

The full harvest

When the foliage has died back, cut the old stems off at the lid. Lift the basket out of the container and tip the medium onto a tray or tarp out of strong sun. Sort through it by hand: the tubers will be in the medium, often tangled in roots and along the old stolons. Handle them gently, because fresh potatoes bruise easily.

Check the skins with your thumb. If the skin rubs off easily, it hasn't set yet. Potatoes you plan to eat soon are fine, but for storage, leave the next container a little longer before you harvest it.

When the harvest is disappointing

• Lots of leaves, few tubers: too much nitrogen late in the season, too much heat in the root zone, or a variety too late for the season.
• Only small tubers: the crop was lifted early, the container ran short of solution, or the plant didn't get enough light.
• Rotten tubers or seed piece: warm solution, a flooded air gap, or a seed potato sitting in the solution.
• Green tubers: light reached the basket. See why potatoes turn green.

Curing and storing

Don't wash potatoes you plan to store. Brush off loose medium, then cure them in the dark at about 60 to 65°F (15 to 18°C) with good airflow for ten days or more, so the skins toughen and small wounds heal. After curing, store them somewhere cold, dark and humid: University of Minnesota Extension suggests 32 to 40°F (0 to 4°C) at high humidity, or 40 to 50°F (4 to 10°C) for potatoes meant for frying, because colder storage makes them sweeter. Eat any damaged or green-tinged tubers first, and keep the rest away from light.

After the harvest

Empty the container and use the leftover nutrient solution to water outdoor plants rather than pouring it down a drain. Clean the container, lid and basket before storing them. Many growers rinse and reuse perlite, while coco coir is usually replaced. Then make a note of the variety, the planting date, when it flowered and how the harvest went, so the next crop starts with better timing.

Sources

• University of Minnesota Extension, "Growing potatoes in home gardens"