Potato

Plant certified seed once the soil reaches 50F

Buy certified seed potatoes, never grocery-store tubers. Warm them at room temperature for about a week until the eyes just begin to sprout, then cut them into blocky pieces with at least one eye each, sterilizing the knife between tubers with 10 percent household bleach. Let the cut pieces heal and cork over for three days somewhere warm and moist, then plant them 3 to 4 inches deep, 12 inches apart, in rows 2 to 3 feet apart, once the soil is at least 50F. Do not use barnyard manure on ground going into potatoes. Cold wet soil stalls the seed piece before it can sprout, and that is exactly when blackleg bacteria and soft rot move in through the cut surface, so the corked-over skin and warm soil let the piece grow away from decay instead of feeding it. Certified seed is inspected for ring rot, which a cutting knife can carry from one infected tuber to the next twenty healthy ones. Manure is the wrong soil amendment here because the common scab organism passes through livestock digestive tracts unharmed and is spread in the manure itself.

⚠ Grocery-store potatoes are treated with a sprout inhibitor and will not grow. Household bleach is corrosive to metal, so wash and oil tools after sterilizing them.

Hold the water steady from flowering through bulking

From the time blossoms form, give the patch about an inch of water a week, delivered as one or two thorough soakings rather than a daily sprinkle. Keep the soil moist but never soggy, and do not let it swing between bone dry and flooded. Flowering is roughly when the tubers begin to bulk, and that is the window in which moisture stress deforms the crop: a dry spell followed by a surge of water restarts growth unevenly and produces knobby tubers and hollow heart. Soggy soil is its own failure, since saturated ground swells the lenticels the tuber breathes through and leaves it open to soft rot in storage.

Hill soil over the tubers before they ever see daylight

Start mounding soil along the row once the stems are about a foot tall, and repeat once or twice more as the plants grow until six to eight inches of soil has been hilled around them in total. Any tuber shoulder that works its way to the surface later needs covering again. Tubers form on stolons just under the surface, and any that catch light turn green and build solanine, a bitter alkaloid that is poisonous in quantity. The same ridge of soil earns its keep twice more: it gives new stolons room to set tubers, and it puts depth between the leaves above and the tubers below, since rain carries late blight spores down through cracks in the soil to infect the crop.

⚠ Excessively green potatoes should not be eaten. Small green portions can be cut out before cooking.

Side-dress nitrogen at hilling, then again two weeks later

About four weeks after planting, as you hill, side-dress roughly 0.15 pounds of actual nitrogen per 50 feet of row. Repeat the same side-dressing two weeks after that. Potatoes require more fertilizer than most vegetables, and the demand peaks while the plant is building the canopy that will later fill the tubers. Feeding at hilling puts the nitrogen in reach of roots exactly when the vines are expanding, rather than after the crop has already decided how much leaf it can afford.

Colorado potato beetle: hand-pick first, and mind the wait if you spray

Hand-pick the adults and crush the orange egg masses on the leaf undersides - on a garden-sized planting this genuinely works. If you spray, Bt tenebrionis and neem hit only small larvae (under 1/4 inch), so scout early; spinosad is the stronger option. Beetles and larvae can strip a plant to stems in days, and the population rebuilds from eggs laid on the undersides of leaves.

⚠ BEES: potato flowers when the beetles are worst, and spinosad and carbaryl are both toxic to bees - hand-pick rather than spray while it is in flower, or spray at dusk once the blossoms close. WAIT 7 DAYS after spinosad or carbaryl before digging - the label PHI applies even though you spray the leaves and eat the tubers. Bt and neem are 0 days, so they can be used right up to harvest. NOTE the number is crop-specific: spinosad is 7 days on potato but only 1 on onion. Many beetle populations are already resistant to carbaryl, so you may take the wait and still get poor control.

Early blight: spray only if it reaches the lower leaves by flowering

As the plants come into flower, check the oldest and lowest leaves for dark target-like spots. If early blight is already showing before or at flowering, spray a chlorothalonil product to protect the healthy leaves above it. If the spotting only turns up on the vines as they mature late in the season, leave it alone. Alternaria solani starts on the oldest foliage and works upward, so an outbreak arriving before the tubers have bulked steals the very canopy that fills them. Early blight almost always shows on maturing vines late in the season, and that late infection does not reduce yield, so spraying then is expense and residue for nothing.

⚠ Do not apply chlorothalonil within 7 days of harvest. It is a protectant with no systemic movement, so it shields healthy tissue but cannot cure lesions that are already there. Copper fungicides give only fair control of early blight.

Late blight: shield the canopy through cool, wet spells

Once the rows close and the canopy starts holding moisture, watch the weather rather than the calendar. When temperatures sit near 65 to 70F with long dews or rain and humidity close to saturation, apply chlorothalonil before any symptoms appear and repeat every 5 to 7 days for as long as the wet weather lasts, covering both leaf surfaces. Pull and destroy any plant carrying greasy, water-soaked lesions. Phytophthora infestans needs a continuous film of water on the leaf to establish, and a closed canopy stays wet long after the open row has dried. Its spores blow in on the wind from tens of miles away, so the first infection is usually not one you could have prevented by sanitation alone. Chlorothalonil has no systemic movement, which is why it only works as a shield laid down before the spores land.

⚠ Do not apply chlorothalonil within 7 days of harvest. It is a protectant only and cannot cure established lesions, so remove infected plants rather than trying to spray them clean.

Kill the vines two weeks before you dig

For any potatoes you mean to store, kill or remove the vines by frost, pulling, or simply cutting them off at the soil line, and leave the tubers in the ground for two weeks before digging. Then dig a test hill and rub a tuber with your thumb: mature skin stays firmly attached, immature skin rubs away. Handle the crop gently and get it out of the light straight away. Those two weeks are when the skin hardens and sets, and set skin is what holds water in and storage diseases out. Cutting the vines also stops a blighted canopy from washing spores down soil cracks onto the tubers below. Do not stretch the wait much beyond two weeks, though, because a longer hardening period increases the black scurf that develops on the tubers.

⚠ Skin that rubs off easily means the crop is immature and will not store, so eat those potatoes fresh instead. Bruises and cuts made during digging are the entry sites storage rots use.

Cure warm for two weeks, then store cold and dark

Hold the harvested potatoes at about 65F for the first two weeks so cuts and bruises can heal, then move them to 35 to 40F in the dark for the rest of storage. If the storage space is not fully dark, keep them in paper bags, which block light while still letting air move. Sort the bin periodically and take out any tuber that has begun to decay. Clear the leftover potato debris out of the garden, and never let cull tubers overwinter there. The curing fortnight lets wounds cork over before soft rot and dry rot can use them as entry points, and the cold that follows slows sprouting and water loss. Darkness is not optional, since even low household light turns tubers pale green and builds solanine. Culls matter beyond the bin: late blight survives on discarded and volunteer tubers, and those piles are the source of next season's first infection.

⚠ Discard heavily greened tubers rather than eating them. One rotting potato left in the bin will spread the decay to its neighbours.