Regional Gardening Reports :: National Gardening Association

In the Garden:
Pacific Northwest
June, 2005
Regional Report

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When potato plants produce flowers, the underground tubers are enlarging. You can harvest new potatoes at this stage.

Digging Buried Treasures

I love poking around in the potato plot. It's a little like digging for buried treasure; you never know the extent of the treasure, but you expect to unearth great things. The leafy aboveground parts of a potato plant are an indication of what's going on down below, and can serve as a guide for watering, fertilizing, and harvesting your spuds.

Seed potatoes are available in a myriad of colors, shapes, and sizes. Since I only have room to grow two or three varieties, I always have trouble deciding which to grow. This year I settled on Yellow Fins because they're reliable producers of fist-sized spuds with a soft and creamy texture; Red Pontiacs, with thin, reddish skin and crisp white flesh; and Nooksak, a russet baker with that crumbly, mealy texture you expect from a baked potato.

Preparing Seed Potatoes
Seed potatoes may be large or small. If the potato is the size of an egg, you can plant it whole. Larger tubers should be cut into pieces, each containing 2 or 3 eyes (indentations on the surface of the spud). Use a clean, sharp knife and cut them up a few hours before planting to allow the cut surfaces to callous over.

Soil Requirements
Potatoes are adaptable, but the ideal potato soil is deep, loose, well draining, yet moisture retentive. I always dig in lots and lots of organic matter prior to planting, to lighten and aerate our clay soil.

How to Plant
I have always had good results with a planting method called hilling. After preparing the planting site by amending with compost, I dig 8-inch-deep trenches and place the potato pieces about 12 inches apart in the trenches. Then I cover them with 3 to 4 inches of soil. Sprouts will emerge in about two weeks, and when they reach 8 inches tall, I gently hill the vine with soil scraped from both sides of the row. I repeat the process two or three times at two-week intervals, each time leaving only 3 or 4 inches of vine exposed. Hilling puts the root system deeper where the soil is cooler and moisture levels remain fairly constant, and the just-scraped soil creates a light, fluffy medium for the tubers to develop into.

Mapping the Progress
Potato plants develop in four phases. During the first phase primary growth from the potato piece you planted begins with an upward-growing stem and results in a canopy of leaves. Depending upon variety and weather, this first phase can last from 30 to 60 days.

As vegetation begins to mature aboveground and starts to set blossom buds, the underground counterparts -- the stolons -- form terminal buds, which will grow into tubers. Development is not dependent upon flower bud formation, but flowers usually indicate tuber development. Adequate moisture is critical during this period.

Blossoming usually marks the start of the third phase -- tuber growth. This happens when leaves produce more carbohydrates than they need for growth, and they send the extra food underground to enlarge the tubers. Too much water during this stage will encourage rapid swelling of the tubers and can result in hollow-heart. It's also during this phase that immature or "new" potatoes can be harvested. A side-dressing of 5-10-10- fertilizer is helpful at this time.

The final growth phase is maturation, when the leaves begin to die back and tuber growth slows. As vegetation dies, moisture and nutrients from the foliage are translocated to the tubers. When the vines have died back about halfway, harvest can begin.

To avoid bruising or breaking the skin of potatoes, it's best to harvest them by hand. If you have a considerable number of plants, a fork can be used to dig the tubers, but set aside any that are injured in the process because they won't store well. Allow potatoes to dry in a protected place for several hours, brush off excess dirt, and store in a cool, dark place.

Potato Problems
Occasional potato problems include the bacterial disease scab, which causes corky lesions on the tuber's surface, and excess exposure to light that promotes greening of the skin. Inconsistent watering promotes scab, so try to avoid it by providing plants with 1 inch of water per week. Adding a layer of mulch to the soil will help retain moisture and suppress weeds, as well. The causes of potato greening are inadequate soil coverage during the growth period, with subsequent exposure to sunlight, and exposure to artificial light during storage.

The concern with greened potatoes is the fact that solanine, a potentially toxic alkaloid, develops in the same area along with the chlorophyll. The bitter taste associated with greened potatoes is caused by the solanine, which is concentrated in, or directly beneath the skin. Peel or cut away green tissue prior to cooking.

Store potatoes in a cool (40 degrees F), dark location, and they'll keep for several months.

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