when to harvest potatoes

When your potato plants fall over, it is natural to wonder if they are dying, and if you will get a potato crop at all. I did some research into what this means, and there are several possible causes. So, why are your potato plants falling over? Potato plants will fall over when the plants are mature and ready for harvest. Potato plants can also fall over if they are too tall due to over fertilization, especially with nitrogen. Temperature, watering, diseases, and pests can also cause your potato plants to fall over, possibly without producing any potatoes. If your potato plants are mature, then it is not a cause for concern when they fall over. However, if they are not mature, then you should investigate why it is happening. In this article, we'll get into the details of what could be causing your potato plants to fall over, and what you can do to prevent it. Let's begin. (If you prefer, you can watch the video version of this article on Youtube). Why Are My Potato Plants Falling Over? One of the most common reasons that potato plants fall over is that they are mature and ready for harvest, so let’s start there. Potato plants turn yellow and fall over at maturity. Your Potato Plants Are Mature & Ready For Harvest When a potato plant reaches the end of its life cycle, the part of the plant above ground will fall over. These shoots and leaves will usually turn yellow before they fall over. So, how big do potato plants get before they fall over? It really depends on the variety. At maturity, potato plant height can range from 12 inches (such as Princess Laratte potato) to 45 inches tall (such as the Red Luna potato). Most potato plants will reach a height of 18 to 24 inches before they start to fall over. You should not cut back your potato plants. Pruning (or "topping") potato plants will limit their growth and potential to produce a good harvest. When they fall over, it is a kind of natural indicator of when to harvest the tubers. You can learn more about how to dig potatoes in my article here. If this happens around the expected maturity date of the plant, there is nothing to worry about. To calculate the maturity date of your potato plants, it is important to keep track of when you planted them. Potato plants will sometimes grow flowers and even above-ground fruit (resembling tomatoes) before maturity! Then, check the time to maturity of the potato variety that you chose. You can find this information online, on the seed packet, on a seed company website, or in the catalog you ordered from. Keep in mind that the times to maturity for potatoes can vary from as short as 75 days (2.5 months) all the way up to 160 days (over 5 months). For example, let’s say you planted Ida Rose potatoes on April 15. Ida Rose potatoes take 95 to 100 days to grow to maturity (let's use 95 days for this example). That means that after 15 days at the end of April, 31 days in May, 30 days in June, and 19 days at the beginning of July, the potatoes would be ready for harvest. So, the potato tubers would be mature on July 19th (15 + 31 + 30 + 19 = 95 days). See the table below for a visual. MonthDays InMonthApril (last15 days)15May (thewhole month)31June (thewhole month)30July (first19 days)19Total95This table shows the number of days in each month to get us to 95 days. Potato plants will sometimes produce flowers or even fruit before reaching maturity. This is a natural stage of their growth, and potato flowers can even help to indicate when to harvest the tubers. You do not need to remove potato fruit from a plant, unless you are worried about pets or children eating them. The fruit that potato plants produce above ground is green and contains toxic solanine, which can be harmful in large amounts. Don't worry if your potato plants fall over and appear to be dying before flowering. It may just mean that your potatoes are ready to pick out of the soil! For more information on times to maturity for potato plants, check out this article on growing potatoes from the Oregon State University Extension. Your Potato Plants Are Too Tall If your potato plants are falling over long before their maturity date, then there is some other problem to deal with. One possibility is that the potato plants have grown too tall. When given too much fertilizer (especially nitrogen), potato plants will grow tall. Overgrown potato plants can get tall due to overfeeding (especially if you use fertilizer that is too heavy in nitrogen). This will promote lots of healthy green growth above ground. However, this will also cause the plant to neglect the production of potatoes, which is the part of the plant you want to eat! For more information, check out my article on over fertilizing your plants, and my article on low nitrogen fertilizers. If your potato plants get too tall, you have the option of using the hilling technique. That means piling up soil around the base of the plant as it grows. For one thing, the extra soil will help to prevent the plant from falling over. For another thing, it will prevent potato tubers from turning green and toxic in the sunlight. You also have the option of staking them (just like tomato plants) to support them as they grow. One option is to use a single stake to support each potato plant. You can use stakes to support potato plants that are growing too tall and falling over. You could also drive stakes into the ground along a row of potatoes and tie off a length of twine between them. Do this at various heights (every 6 inches) and let the potato plants climb as they grow. For more information, check out my article on how to support plants. Temperatures Are Too Extreme For Your Potato Plants If you can rule out over fertilization of your potato plants, then it is time to consider extreme temperatures. This would be more of a problem in containers, where the soil is not as well insulated as the ground. A potato plant in a container is more likely to see extreme temperatures. Potato tubers (the part of the plant you eat!) form best at temperatures between 60 and 70 degrees Fahrenheit (16 to 21 degrees Celsius). Potato tubers will fail to form when the soil temperature is 80 degrees Fahrenheit (27 degrees Celsius) or warmer. So, avoid planting them too late in the season, when they will encounter warmer soil temperatures. It is best to plant potatoes as soon as the soil temperature is 45 to 50 degrees Fahrenheit (7 to 10 degrees Celsius). Potatoes can tolerate light spring frosts, but if temperatures get too far below 32 degrees Fahrenheit (0 degrees Celsius), the plant will die and fall over. If a fall frost threatens your potato plants, you can protect them with row covers by wrapping the whole plant down to the soil. For more information, check out this article on potatoes from the University of Illinois Extension. Improper Watering Of Your Potato Plant If your plant has not encountered any frost or freezing, then it could be that improper watering is causing your plants to fall over. Be careful not to over water or under water your potato plants, or else you could kill them! If you over water your potato plants, the soil becomes waterlogged. At that point, the roots will not get enough air from the soil. Eventually, the roots will rot, which will keep your plant from absorbing water and nutrients. Your potato plant will not survive long in this state, and will soon fall over. If you under water your potato plants, their leaves will sometimes curl as an early indication. The best way to check whether your plant needs water is to feel the soil with your fingers. If the soil is dry to a depth of a few inches, then it is time to water your plants. When you do water your potato plants, water from below, to avoid wetting the leaves. If you need to use a sprinkler and get the leaves wet, water in the morning or at midday. This will leave time for the leaves to dry off before nightfall, which decreases the chances of fungal disease. Water deeply and infrequently, rather than providing a shallow amount of water on a frequent basis. It is also recommended to avoid watering potato plants until after the plant emerges from the soil. For more information on watering, check out this article on growing potatoes from the Oregon State University Extension. Your Potato Plants Have A Disease After ruling out over fertilization, extreme temperatures, and improper watering, we now need to consider the possibility that your potato plants are infected with a disease. Unfortunately, there are many diseases that can affect potato plants and devastate your crop. One such disease is blight, which comes in two types: early blight and late blight. Your Potato Plant Has Early Blight Early blight of potatoes is caused by a fungus called Alternaria solani. This fungus spreads by a few different methods: spores carried on the windhuman contactwater splashing infected soil onto the lower leaves of a plant Here, we can see symptoms of early blight: brown spots on leaves, with yellow around the spots. Early blight will cause the leaves of potato plants to get brown spots with yellow rings on the outside of the spots. This happens on the older (lower) leaves of the plant first. A severe early blight infection can cause fewer and smaller potatoes growing on your plant. It can also cause lesions on the potatoes themselves. For more information on potato early blight symptoms, check out this article on early blight of potatoes from the Michigan State University Extension. Early blight will spread readily in damp conditions. An outbreak of early blight will often follow extended periods of rainy weather. The fungus that causes early blight can survive over the winter in the soil. It can then infect newly planted potatoes the following spring. This is more likely to occur if you use compost that contains infected plants, and if the compost pile did not get hot enough to kill the fungus that causes early blight. Early blight can also get into your garden if the seed potatoes you used were infected to begin with. Make sure to order clean seed potatoes from a reputable seed company that guarantees their products! Finally, remember that early blight also affects other nightshade vegetables, such as tomatoes, peppers, and eggplant. So, an infection of any of these plants can also spread to your potato plants. Thus, it is a good idea to practice crop rotation and avoid planting any of these crops in the same area two years in a row. You can learn more about crop rotation in my article here. Your Potato Has Late Blight Late blight of potatoes is caused by a fungus called Phytophthora infestans. This fungus spreads by spores on the wind. This potato has been infected with late blight. I don't want to eat that one! You shouldn't plant infected tubers either. Late blight is easily the more devastating of the two types of potato blight. This disease can spread rapidly between plants. Left unchecked, late blight can destroy entire crops of potatoes. In fact, late blight was responsible for the Irish Potato Famine of the 1840’s. Late blight causes large, dark brown blotches on the leaves and stems of infected plants. Over time, the tubers may also be affected. Late blight cannot survive in isolation in the soil. However, it can survive over the winter by "hiding" in these infected potato tubers. As a result, you should remove any “volunteer” (unplanted) potato plants that grow in your garden, as a precaution against the spread of late blight. Late blight spreads rapidly in damp weather at cool temperatures from 60 to 70 degrees Fahrenheit (16 to 21 degrees Celsius). Finally, remember that late blight can also infect tomatoes. The wind can help the spores to carry the disease from your tomato plants to your potato plants. There is currently no cure for late blight in potato plants. Your best bet is to remove infected plants before the disease can spread to any others in your garden. For more information, check out this article on late blight from the University of Minnesota Extension. Other Potato Diseases Two other common diseases that can affect potato plants are fusarium wilt and verticillium wilt. Your best bet is to purchase seed potatoes for varieties that are resistant to both of these diseases. In seed catalogs or on company websites, disease resistance will often be denoted by an F (for fusarium wilt) and a V (for verticillium wilt). Pests Are Attacking Your Potato Plants If no diseases seem to be present in your potatoes, then it is time to check for pests. A couple of common pests that can cause problems for potato plants are cutworms and Colorado potato beetles. Cutworms Are Attacking Your Potato Plants Cutworms can chew and completely cut off your potato plants at the stem, close to the ground. If the potato plants that fall over are completely detached, then you likely have cutworms roaming in your garden. Cutworms will chew the stems of potato, tomato, and other plants. Is it possible for them to sever many plants in one night! As any tomato or potato grower will tell you, a good way to prevent cutworm damage is to use cutworm collars on your potato plants. For more information on cutworm collars and other tricks, check out my article on how to get rid of cutworms using natural methods. Colorado Potato Beetles Are Attacking Your Potato Plants The Colorado potato beetle has an orange or yellow body, with black stripes running along its back. They are about a quarter of an inch to half an inch in length. Colorado potato beetles can multiply quickly and cause damage to your potato plants. A single Colorado potato beetle can lay up to 500 eggs in a 5-week period. These eggs are usually found on the underside of a leaf. Colorado potato beetles are one of the biggest culprits in potato plant defoliation (eating and removal of the leaves). If there is enough leaf damage, you could certainly see your plants falling over and dying before maturity. Over the years, Colorado potato beetles have developed resistance to many pesticides. Crop rotation is one way to prevent them from spreading. Using straw as mulch for potato plants also helps to hide the potato plants, and encourages the potato beetle’s predators to protect your plants. You can learn more about Colorado Potato Beetles in this article from the University of Minnesota Extension. Conclusion The table below gives a brief summary of the reasons why a potato plant might fall over and what to look for. CauseSignsPlant readyfor harvestPlant is healthy up untilmaturity date, and mayproduce flowers or fruit.Plant over fertilized(especially nitrogen)Plant is healthy and hasplenty of green growth,but possibly no flowers,fruit, or tubers.ExtremecoldPlants may fall aftera hard frost (spring/fall).OverwateringSoggy soil that results inroot rot. Watering doesnot seem to help. Maysee some yellow leaves.UnderwateringDry soil, wilted plant,and yellow leaves.DiseaseYellow, brown, or blackspots on leaves or vines.Pests(cutworm)Plants are completelydetached from stems,cut off at soil level.Causes and signs of a potato plant falling over. Now you have a much better idea of why your potato plants are falling over. Hopefully, it is due to the fact that your potatoes are mature and ready for harvest! However, if that is not the case, then you know what other problems to look for. You also know how to prevent these issues from hurting or killing your potato plants. You can learn how to plant sprouted potatoes in my article here. You might also want to check out my article on how to dig (harvest) potatoes. I hope you found this article helpful – if so, please share it with someone who can use the information. If you want to read some of my most popular posts, check out the “Best of GreenUpSide” page here. Enjoy! ~Jonathon