We put potatoes with our other large-space crop, sweet corn, in the fourth part of our rotation. One reason is that potatoes work well as a companion to peas, if grown in alternate beds with them. As the peas grow, so do the potatoes, and hilling the potatoes creates additional room down the rows for pea harvesting. It also puts them two years away from their close cousins, the tomatoes, eggplants, and peppers.
The first step is to prepare the seed potatoes for planting. The ideal size piece is two or three ounces; large potatoes can be cut into two or three sections as long as each piece contains at least two eyes (the small depressions on the surface with buds in them, from which sprouts will grow). Cut the “sets” a day before planting, so that the cut inner surfaces have a change to callus over; that, and dusting with powdered sulfur will help prevent rot from setting in while they are underground starting to sprout. Some gardeners like to let the potato pieces sprout before planting, but I’ve found it slows down the planting too much, as you’ve got to be overly careful not to harm the sprout during planting.
It’s essential that potatoes be planted in well-drained soil, as the crop is borne underground. To give our spuds plenty of fertile, friable soil to grow in, we plant a single row down the center of a three-foot bed. Remove a single shovelful of soil every foot; there’s no need to measure—just put the holes right next to each other and the spacing will be right. Then take the prepared sets and place one in the bottom of each hole. If you’re in a hurry or have a lot of planting to do you can just toss the sets in the holes, and they’ll survive; but by putting them carefully in the bottom, with the eyes pointing up, you make their job easier.
After placing the sets, we put a shovelful of compost on top of each—enough to cover it—and then fill the holes back in with the soil we removed. We often interplant a quick crop down both edges of the bed, say lettuce from transplants, or maybe spinach or arugula.
That’s it until the sprouts break ground. Once they do, there are two ways to proceed: hilling or mulching. The potato is actually a tuber, or stem offset of the plant, and to prevent the formation of mildly poisonous alkaloids in its skin (indicated by greenness), it must be kept completely dark. As the stem grows, more and more tubers form, so more soil or mulch needs to heaped around the base of the plant over the course of the season. If you start by hilling, you can switch to mulching later; however, the reverse is not true, as once you begin to mulch you cover the soil that would otherwise be used for hilling.
The other problem with mulching is that straw, the ideal material, is expensive in most parts of the country, and hay—unless you cut it yourself, before the grasses from which is made are mature—will spread weed seeds all over the garden and lead to a cultivation nightmare the following season. On the positive side, though, is the fact that a deep straw or hay mulch will help keep the soil cool, which potatoes really like. We are fortunate to have a quarter-acre in clover each season from which we can cut mulch, but if you don’t have the room to grow mulch I’d recommend hilling instead of using purchased hay.
The first hilling (or mulching) should be done when the plants are six to eight inches tall, at which time you harvest whatever crop you may have planted on the edges. The idea is to force the plant to grow taller, so that there is as much room along the underground part of the stem as possible for potatoes to form. Soil should be brought as far up the stem as possible without actually covering any leaves: they provide the fuel for further growth!
Over the course of the season we hill three times: first, we run a stirrup hoe down both sides of the bed, outside the potatoes and including the slopes of their hills; then, using the loosened soil produced by this cultivation, we raise the hills. The simplest method we’ve found for doing this is to stand in the path on one side of the row and, reaching across with a broad hoe, pull the soil from the other side toward us (and the potatoes). That way we can work quickly along the row, stepping sideways after each plant is hilled. After the third hilling you might want to spread a light mulch, if you haven’t already—to shade the soil and prevent further weed encroachment.
Pests & Diseases
The most serious pest of potatoes is the Colorado potato beetle (CPB to the initiated), a 1/2-inch-long, bulge-backed little devil that is yellow-orange with a series of black stripes running from front to back, and a mass of black spots on its head.
The major diseases of potatoes are blight and scab. The scab fungus exists in most garden soils, but it can be counteracted through good crop rotation and by adjusting the level of soil acidity (fungus growth is promoted by a high pH, that is, low acidity). In areas of the country with naturally acidic soil, pH can be kept low by not liming the section of the garden where potatoes will grow. A fall cover crop of annual ryegrass also discourages scab and adds fresh organic matter to the soil, thus increasing the activity of competing soil microorganisms. Scab is worse in dry soil, so make sure that your potatoes receive regular irrigation if rainfall is less than an inch a week.
Late blight is also a soil-borne fungus, though it can also be carried over on tubers saved for future planting. It is brought on by moist conditions, and can spread rapidly once established, killing a whole planting of potatoes outright. The symptoms are sunken, blackish areas at the margin of older leaves, which then wither and fall; eventually the whole plant is affected. An outbreak often follows a hot and humid spell of weather which breaks to a cool, clear night below 60˚ F (16˚ C) with heavy dew, and then warms up again. This is a common weather scenario where I garden, and our potato plantings are quite likely to develop the late blight. It can be prevented by spraying the plants with a copper solution during susceptible periods, though I don’t like putting all that copper on my garden. Blight usually kills the plants only after they have already set a good crop of potatoes. Rotation also helps with control of late blight, by keeping new crops away from a source of infection. Mulch, by preventing soil splash during rainy spells, may also slow its growth.
Harvesting of potatoes is usually left until the end of the season, but with most varieties you can start digging a few small, “new” potatoes as soon as the plants flower—if they flower. Many varieties never do. My recommendation is to plant a few hills of an extra- early, moist-fleshed variety and use those for fresh harvest; then the higher-yielding, better-storing types can be left undisturbed.
The proper curing and harvest of potatoes is an imprecise art. At maturity the skins of potatoes are thin and tender, easily hurt during harvest, which shortens their storage life considerably. The trick is to let the tops wither and die, then let the tubers cure for a week to ten days before gently unearthing them. During this time the skins will thicken.
If the tops die from disease rather than old age, though, you should remove and burn them; then—if the weather stays dry and reasonably cool—wait until frost threatens to dig the potatoes themselves. If rain or hot weather is in the forecast, the waiting period can be trimmed to two weeks, just long enough for the disease spores (left on the soil surface by removal of the tops) to die. That way the spores won’t infect the potatoes themselves during digging.
Once you’ve got the potatoes out of the ground they will benefit from an additional week spread on the floor of a garage or shed—a cool but frost-free, totally dark location. During this period the skins will firm up further, and the sugars within the tubers will change to starch, which completes their ripening process. Also, any soil left on them will dry, and at the end of this time they can be lightly brushed off before being put into storage. Don’t wash them or let them get wet after harvest.