Care Tips For Your Potato Plants In Late Summer For A Bountiful Fall Harvest, From Our Pro Gardener

Growing potatoes in the garden is fairly easy, and they can also be grown successfully in containers. I've grown them both ways, and have advised my clients on both methods and which varieties to choose, but I prefer growing them in the ground if I have space. These tasty vegetables can be somewhat vulnerable to various pests and diseases, however. The Irish potato famine was caused by a disease known as potato blight, which made potatoes inedible and wiped out entire crops. Knowing a few tricks and tips for late summer care can help you keep your potato plants healthy and maximize your harvest. 

The most important tasks for thriving potato plants are hilling up soil around plants, watering regularly, and preventing or controlling pests. Most gardeners plant potatoes in the spring. I try to start mine in April, but they can be planted as late as June for an autumn harvest. The time span from planting to maturity varies widely depending on the variety (fingerlings, despite being small, can take up to 130 days to mature), and mature size is sometimes dependent on factors like soil and rainfall.

Following these care tips will help increase your chances of a good potato harvest. It's also important to realize that some potato varieties are easier to grow than others, so if you're a beginner, choosing to grow a good foolproof variety like 'Yukon Gold' or 'Dark Red Norland' will help you gain some potato-growing confidence before trying slightly more fussy varieties.

"Hilling up" soil is an important step in potato growing. This means adding an extra layer of soil on the tops of the plants to protect the tubers and roots. When the plants reach 8 to 10 inches high, use a hoe or small rake to gently mound up soil around the base of the plant. Leave the top 3 to 4 inches of leaves exposed, and avoid disturbing the roots. Hilling up ensures that potato tubers receive ample nutrients and gives them space to put on size. 

But more importantly, this practice protects potatoes from sun exposure, which can cause "greening," a form of discoloration that makes potatoes inedible. You should hill up soil twice: Repeat two to three weeks after the first time, and then just check to make sure the soil stays in place. As summer progresses, hilled soil may show disturbance after wildlife visits or heavy rain. If so, gently replace the soil and tamp it down lightly. 

Adding a thin layer of lightweight mulch (like straw, leaves, or a bit of shredded pine bark) after the second "hilling up" is also recommended. This helps to keep the mounded soil intact, regulates moisture, discourages beetle infestations, and as a bonus, can help to protect your potato plants from heat or frost damage. 

In addition to making sure potatoes are planted in full sun to feed their leaves and hilling up soil to protect the tubers, water is another important element. Potatoes need ample water to grow to maturity. If rainfall is inadequate in summer, potatoes may stay small, so it's important to maintain a regular watering schedule for your potato plants. One inch of water per week is a good goal to aim for, but keep an eye on the rain forecast to avoid overwatering.

Potatoes don't do well in either extremely dry or extremely wet soil. Adding amendments like compost can help soil retain moisture, as can mulching. If there's heavy rain on the way, you may want to protect your potato plants to prevent having soil washed away; placing some burlap sacks over them can help with this, or you can use old towels or bedsheets.

Consistent watering throughout the entire summer is crucial to make sure your potatoes grow to their proper size. By August, some early-maturing varieties may be ready for harvest. In my area, rainfall tends to be more sparse in late summer, so I try to make sure I give my potatoes plenty of supplemental water. Don't let the plants dry out between waterings: Keep them slightly moist by watering lightly every day, especially during a drought.

There are a few pests that like to devour potato leaves and tubers. One of the worst is the Colorado potato beetle, a small bug that actually looks a bit like a potato. One simple way to protect your plants from these destructive pests is to mulch the plants with a light layer of straw or dried leaves. This protects the vines and provides habitat for beneficial insects that devour the beetle's larvae. These beetles should be quickly removed by hand if you see them: toss them into a container of vinegar. Sorry, beetles, no potatoes for you.

For other leaf-eating pests like leafhoppers, organic sprays such as garlic spray can be effective. Aphids also like to eat potato plants, but these can be fairly easy to prevent if you add some nearby plants that attract beneficial insects who eat aphid larvae, like ladybugs and lacewings. Four-legged creatures can also be an issue. I've had woodchucks plow into my potato patch and uproot plants, though they don't actually eat the potatoes. Still, some short wire fencing or other kind of baffle (like wooden dowels with string) will usually keep them away. Smaller rodents like voles can also be a problem. They prefer to go after larger potatoes, so harvesting early on will help prevent your crop from being targeted by these rodents.

Watch your plants for signs that potatoes are ready to harvest. Two or three weeks after blossoms start to fade, you can harvest new potatoes, which refers to younger, smaller potatoes of any variety. Dig them gently, as these young potatoes have thin skins and are easily dented with tools. Most potatoes take at least 90 days from planting to mature to full size, and some take up to 130 days. Pulling up vines or digging with tools to "check" the size might damage the tubers or vines, and interrupt growth. 

Though it may seem odd, the surest sign that potatoes are ready to harvest is when the plant looks like it has died. This is assuming it hasn't died prematurely from disease or lack of water. When the mature plant's stems droop and turn brown, wait at least a week to let potatoes cure, then dig them up. Use a small spade (or a potato hook, if you have potatoes planted in rows) and be gentle, as digging too vigorously may cut or split potatoes.

Generally, it's best to harvest potatoes before frost to prevent mushiness, though most varieties can handle a light frost (29-32 degrees Fahrenheit) without much damage. Keeping an eye on the nighttime temperatures will help let you know when there might be a danger of hard frost, and bear in mind that temperatures can plummet suddenly, so stay alert and allow some time for an "emergency" potato harvest.