growing potatoes and tomatoes together

Since potatoes (solanum tuberosum) and tomatoes (solanum lycopersicum) are the in the same family (nightshade), it makes sense to ask if they can be grown together. I did some research to see if this is a good idea, and I found some interesting information. So, can you grow potatoes and tomatoes together? No, you should not grow potatoes and tomatoes together. While they are both in the nightshade family, potatoes and tomatoes have different requirements for soil pH. There are also some diseases, such as early blight and late blight, which are common to both plants and can be spread between potatoes and tomatoes by insects that attack both plants. Even though you should not plant potatoes and tomatoes together, you can still have both in your garden, provided that you take measures to prevent the spread of disease. Let’s take a look a closer look at the proper growing conditions for each plant, the common diseases for both, and how to discourage these diseases in your garden. Reasons Not To Plant Potatoes and Tomatoes Together There are several reasons not to plant potatoes and tomatoes together. Let’s start with one of the biggest reasons: soil pH. Soil pH Tomatoes prefer a slightly acidic soil pH in the range of 6.0 to 6.8, similar to most other garden plants. Tomato plants prefer soil that is only slightly acidic, with a pH of 6.0 to 6.8. Potatoes, however, prefer a fairly acidic soil pH of 4.8 to 5.5. Potato plants prefer a more acidic soil, with a pH from 4.8 to 5.5. Remember that pH is measured on a logarithmic (exponential) scale, so a pH of 5.0 is ten times as concentrated (acidic) as a pH of 6.0. Obviously, one or both plants will be unhappy if you try to grow both in the same soil. You may be wondering why soil pH is so important for plants. Just remember that the availability of nutrients in the soil depends on the soil pH. For more information, check out this article from Research Gate, which shows the relationship between soil pH and nutrient availability. For instance, at a pH of 4.8, calcium is starting to become less available in the soil. Potatoes might do fine in this type of soil, but tomatoes may end up with blossom end rot, a disease caused by a lack of calcium. Blossom end rot in tomatoes is caused by a calcium deficiency. Even worse, this can happen even if there is plenty of calcium in the soil! Remember that in this case, nutrient deficiency is caused by a pH imbalance, not by a lack of nutrients in the soil. In short, the difference in pH preference alone is reason enough to plant potatoes and tomatoes separately. Growth and Harvesting Even if you managed to get potatoes and tomatoes to grow together in the same soil, you might have trouble growing them to maturity without damaging the tomatoes. Remember that potatoes have leaves above ground, but the potato tuber itself grows underground. An early potato harvest can damage the roots of tomato plants if they are planted too close together. Planted too close to tomatoes, the potatoes can grow large enough to impede the roots of tomato plants. You may also damage a tomato plant’s roots when harvesting nearby potatoes. Also, remember that if potatoes and tomatoes are grown close together, there is a much higher chance that disease will be transmitted between the two plants. Diseases Common To Potato and Tomato Plants There are two serious diseases common to both potato and tomato plants: early blight and late blight. Let’s take a closer look at both diseases, including signs and symptoms for each plant. Early Blight Early blight is a common disease for tomatoes, affecting their stems, leaves, and fruit. Despite the name, early blight usually only appears on mature plants. Symptoms include severe defoliation (loss of leaves) and scarring of fruit. The spots on fruit are black and leathery, and severely infected fruit may fall from the plant. For potatoes, signs of early blight include small, dark brown spots on older (lower) leaves. The disease can cause fewer and smaller potatoes. The disease may also cause lesions on potato tubers themselves, with parts underneath becoming like leather or cork. Early blight in both potatoes and tomatoes is caused by the fungus alternaria solani. A closely related fungus, alternaria tomatophila, can also cause early blight in both plants. However, alternaria tomatophila is the more common cause of the disease for tomatoes. It is also worth noting that both fungi can also infect eggplant, which you may not want to plant close to tomatoes or potatoes in your garden. Peppers, along with eggplants, are also in the nightshade family, along with potatoes and tomatoes. Generally, early blight develops in warm temperatures, from 59 to 80 degrees Fahrenheit. High humidity levels (90% or greater) also encourage growth of early blight, since it is a fungus. Remember that both alternaria solani and alternaria tomatophila can survive the winter in soil, compost, or other organic matter. This means that soil may need to be left unplanted for many years to eliminate the disease. Even worse, early blight can reside in infected potatoes, so if you buy potatoes that are infected and try to plant them after they sprout, you can end up introducing early blight to your garden. It is also worth noting that you should not compost infected plants. Instead, burn them or dispose of them immediately. Since the spores can be spread by wind, rain, irrigation, machinery, or human contact, it is important to identify infected plants early and remove them quickly to prevent the spread of early blight in your garden. To prevent early blight in the first place, choose disease-resistant varieties of potatoes and tomatoes in the future. According to the University of Minnesota, some tomato varieties that are resistant to early blight include Iron Lady, Mountain Supreme, and Mountain Magic. Also, avoid irrigation in cool, cloudy weather, and water early to allow plants to dry before nightfall. The longer the leaves stay wet, the better the chance of fungus growth, and thus the more opportunity early blight has to take hold. For more information, check out this article from the University of Minnesota on early blight in tomatoes and this article from Michigan State University on early blight in potatoes. Late Blight Late blight can affect leaves, stems, and fruits of tomato plants. It can spread very quickly and can be devastating to gardens. The disease was responsible for the Irish potato famine of 1840. The oomycete (fungus-like organism) Phytophthora infestans is the cause of late blight. The symptoms include large, brown spots on leaves and stems. Fruits may also develop firm, brown spots. Eventually, other bacteria can invade the weakened fruit and cause mushiness. When humidity is high, powdery white fungus appears on infected plants. With severe late blight infections, a foul odor may be detected. Late blight will spread most at temperatures between 60 and 70 degrees Fahrenheit, during periods of high humidity. The disease can survive over the winter, but is not as likely to do so as early blight. In light of this, do not compost any infected plants. Instead, burn them or dispose of them immediately. Late blight can produce thousands of spores in less than a week, potentially infecting your entire garden’s potato and tomato crop, along with those of neighboring gardens. The best way to avoid late blight is to cultivate disease-resistant varieties of potatoes and tomatoes. According to the University of Minnesota, tomato varieties that are resistant to late blight include Mountain Magic, Plum Regal, and Iron Lady. You should also inspect established tomato plants that you purchase, in order to ensure that they are not infected with late blight. Also, be sure to water at the bottom of plants – avoid getting the leaves wet, and avoid watering late in the day so that the plant does not stay wet overnight. For more information, check out this article from the University of Minnesota on late blight and this article from the University of Maryland on late blight. How To Use Crop Rotation to Prevent Disease In Your Garden The best way to discourage early blight, late blight, and other diseases in your garden is to employ crop rotation. Crop rotation simply means that you do not plant the same crop in the same location two years in a row. Sometimes, crop rotation calls for 3 or even 4 year rotation cycles. For instance, you might plant leaves (lettuce), fruits (tomatoes), roots (carrots), and legumes (peas) in each of the four years in your crop rotation cycle. Including peas in your crop rotation can help to prevent disease - they also help to add nitrogen to the soil. Even though potatoes (roots) and tomatoes (fruits) are in different categories, they should not be planted in the same area in successive years. If you want to grow all 4 common nightshade plants (potatoes, tomatoes, peppers, and eggplant) in your garden, here is a crop rotation method you can use. Peppers are another good plant to add to your crop rotation. Like potatoes and tomatoes, they are in the nightshade family. First, split your garden into 16 equal sections: 4 rows by 4 columns. Assign one plant to each row, depending on growing requirements such as sunlight and pH (remember our discussion of soil pH earlier!) Be sure to keep the tomatoes and potatoes far apart to prevent the spread of disease between the two. For instance: Row 1 – tomatoes Row 2 – peppers Row 3 – eggplant Row 4 - potatoes Then, assign a different column to each plant. For instance: Column 1 – tomatoes Column 2 – potatoes Column 3 – eggplant Column 4 – potatoes Plant each crop in the correct section, based on its row and column. Every year after that, you should move each plant one column to the right. (If a plant is already at the far right, move it to the far left). This prevents the spread of early and late blight between potatoes and tomatoes, and ensures that there are four years between plantings of these crops in the same area. Conclusion By now, you should have a better understanding of why it is a bad idea to plant potatoes and tomatoes too close together. Not only do they require different soil pH levels, but they also share diseases that can spread between the plants easily. I hope you found this article helpful – if so, please share it with someone who will find the information helpful. If you have any questions or advice of your own on potatoes, tomatoes, and crop rotation, please leave a comment below.