Potatoes
Vegetable Fact Sheet
(printable version)
Potatoes are tubers and a cool season plant. The edible part is the tubers (potatoes) growing on the underground stems.
Sources for seed potatoes
Local nurseries and seed catalogues. Try Ronniger’s Potato Farm at www.ronnigers.com; Territorial Seed at www.territorialseed.com or Nichols Garden Nursery at www.nicholsgardennursery.com. These examples do not imply an endorsement of any particular supplier from WSU Extension, but are listed as examples of where you can look for seed stock and more information of varieties available each year.
Varieties
Fingerlings, russets, yellow potatoes, blue potatoes, red potatoes. Old standbys are available but new varieties are developed and perfected each year.
Soil Preparation
Find a well-drained sunny spot in your garden. Sandy loam is ideal. Potatoes require a well-balanced soil of 6.0 to 6.8 pH. Alkaline soil will produce scab. Potatoes thrive on newly cultivated ground. Dig the area up well and incorporate organic matter. The deeper it’s dug up the better. Go very easy on additional fertilizers. Remember to plant cover crops each year in all areas of your vegetable garden to renew the nutrients used up by growing vegetables each year. Cover crops not only return nutrients to the soil, but they keep weeds to a minimum.
When to Plant
Plant as early as March (for harvest during summer months and into fall) and as late as mid-June (for late fall harvest). Some gardeners say, “Plant when the dandelions begin to flower.” In a wet spring first plantings sometimes rot. Try again in late spring or early summer if this happens.
How to Plant
Select certified seed potatoes (these are grown and certified to be disease-free). Look for scab-free varieties. Cut seed potatoes into sections making sure each section has two or three eyes (growth buds). Let the sections dry for several hours or a day so the cut parts callous over.
The width between rows depends on the size of your garden. Farmers need 30-36 inches between rows. Gardeners can use 20-26 inches between rows. Dig a trench, mounding the soil up on either side. Place the seed potato sections in the trench. Use a rake and cover the seed pieces immediately with no more than 4-inches dep. After abut two weeks you’ll see the new plants emerge (depending on how warm temperatures have been). When the stems are about 8-inches tall, gently hill with the soil from the sides of the trench you dug. Potatoes (tubers) grow along the stem of the plant; to maximize yield and prevent sunburn of the potatoes, keep covering up the stem with soil or straw until the stems are covered from six to eighteen inches deep. As a general rule, hill the potatoes about three times spaced two or three weeks apart. The more you hill up the potatoes the higher your yield will be. The hills also maximize the heat from the plant too.
Watering
Keep soil evenly moist. Weed regularly. Continue mounding up soil around vines as vines grow to keep growing tubers protected from sunburn.
Mulch Method
If you soil is shallow, rocky or not as ideal as desired, try hilling your potatoes with dry straw to cover up the tubers as they grow along the emerging and growing stem.
Cage Method
Grow potatoes in vertical boxes, cribs, barrels, wire cages or garbage cans with holes drilled for good drainage. Start with 6-8 inches of good soil and as the plants grow, begin adding compost, mulch or soil. As the plant stems lengthen, the covered up portions will produce more and more tubers. You’ll grow more potatoes in less space and the yield will be equal or more than other methods. Cages require careful monitoring because water can evaporate rapidly.
When to Harvest
Potatoes will be ready to harvest in 90-120 days after planting. Harvest tender “new” potatoes when vines start to flower. Reach down into the hills and carefully remove the potatoes you desire and then recover the stem so more potatoes can grow. (New potatoes are to be coked and eaten quickly; they don’t keep over a long time). Harvest mature potatoes two weeks after tops of the plants die.
How to Harvest
About two weeks after the plants die dig beneath the plants with a spading fork or shovel. Dig around the plant about 8-10 inches away from the plant to avoid puncturing the potatoes. If you puncture or slice into a potato, place it aside from the other potatoes; wash it well and cook as soon as possible to avoid rot. Lift the plant gently, shake off the loose soil and pull the potatoes from the vine. Make sure you sift through the area around the plants because you’ll find even more potatoes than those you‘ve harvested “on the vine.” Extra potatoes pull away from the buried stems and you’ll find them in the soil. It’s like a treasure hunt.
Storage
Late-maturing potatoes are best for storage. Dig potatoes for storing on a dry day. Spread the potatoes on a dry surface until the soil has dried and fallen off the skin. Do not wash the potatoes before storing. Do not leave them in the sun (white potatoes turn green when exposed to the light and the green color indicates solanine which is poisonous eaten in large quantities). Store potatoes in the dark at 35-40 degrees. Use a slatted bin or large mesh bags raised one or two inches off the floor. Because they need air circulation do not pile them deep than 12-18 inches. This method should prevent rotting, softening and early sprouting.
Problems
Scab – Tubers with corky scabs on skin surface. Russet varieties are more resistant than smooth-skin varieties (Nooksak is resistant). Scab is more a problem in alkaline soils or soils with high organic matter. Do not use wood ashes. Peel off scabby skins or cover with foil and bake.
Flea Beetles – If you see numerous pin-sized holes in leaves or in the skin of the potatoes it may be from the shiny black beetles called Potato Flea Beetles. Potatoes are safe to eat – peel away the damage on the skins.
Late Blight – Leaves with large dark green to purplish water-soaked areas; during moist weather (in late season) moldy growth develops on undersides of leaves; vines die as disease spreads rapidly; tubers in storage develop shiny metallic, purplish sunken areas; area beneath skin is discolored about ¼ -inch; soft rots may follow. If blight is a problem in your area cut vines to one-inch below soil surface and remove 10-14 days before harvest prevent infective the tubers (the potatoes). Use resistant varieties such as Kennebec or Nooksak. Fungicide will aid if applied prior to summer rain or when symptoms first noticed.
Wireworms (stiff, waxy yellow wireworms in soil) – Tubers with 1/8 to ¼ inch holes and tunnels. Harvest early potatoes as soon as possible. Damage will be less if late potatoes are planted in late June.
Slug damage – Tubers with large gouges or holes. Use iron phosphate (Sluggo is one of the brand names of this organic and safe products). Carefully read and follow label directions.
Information compiled from Washington, Idaho, Colorado and Virginia Cooperative Extension Websites. Aslo from Ronninger’s Website, Sunset Western Garden “Vegetable Gardening,” and WSU Extension Washington Bulletins EB0422, EB0968, and EB 1326.
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