Crop rotation is a fundamental strategy in gardening, especially after growing a demanding crop like potatoes. Potatoes spend an extended period in the soil, significantly depleting specific nutrients, most notably potassium. Furthermore, the potato crop can leave behind a concentrated environment of pests and pathogens. To prevent the buildup of these issues and ensure the continued health of the garden bed, the subsequent planting must be carefully considered.
The Nightshade Family Restriction
The most important rule of post-potato planting is to avoid members of the nightshade family (Solanaceae). This restriction is based on shared vulnerability to diseases and pests that can overwinter in the soil. Planting another nightshade immediately after potatoes guarantees the next crop faces a high risk of infection.
Specific crops to avoid include tomatoes, peppers, and eggplants. These plants are highly susceptible to the same fungal organism, Phytophthora infestans, which causes late blight. This pathogen survives on infected plant debris left in the soil, and a new nightshade crop provides an immediate host for the disease to flourish.
Sharing a family tree also means sharing pests, notably the Colorado Potato Beetle. The beetle’s eggs and larvae thrive on all Solanaceae species and readily move from potato remnants to a newly planted tomato or pepper crop. Even ornamental nightshades, such as petunias and tobacco, should be excluded. A minimum rotation period of three to four years is recommended before planting any nightshade back into the same spot.
Crops Susceptible to Soil-Borne Pathogens
Beyond the nightshade family, gardeners must consider other crops highly vulnerable to the soil-borne pathogens potatoes commonly leave behind. These organisms persist in the soil and can infect the new crop’s roots or storage organs. Choosing a non-resistant crop perpetuates the disease cycle, worsening the problem for future plantings.
One major concern is Common Scab, caused by the bacterium Streptomyces scabies. This pathogen infects potatoes, but it also readily attacks other root crops, causing characteristic corky lesions on their surface. Susceptible crops include beets, carrots, parsnips, and radishes. It is advised to avoid planting beets directly following a potato crop due to this shared vulnerability.
Another persistent threat is Verticillium dahliae, the fungus responsible for Verticillium Wilt. While nightshades are severely affected, V. dahliae has an extremely wide host range, encompassing over 140 plant species. This means non-nightshade vegetables such as cabbage, broccoli, rutabaga, and other root crops like beets are also susceptible.
Avoiding Root Vegetables and Heavy Feeders
The physical and nutritional condition of the soil after a potato harvest is an important factor in determining what not to plant next. Potatoes are heavy feeders, removing a large amount of nutrients from the soil during their growth cycle. They have a high demand for potassium, which is essential for tuber formation.
Planting another heavy-feeding crop immediately afterward is discouraged because the remaining soil often lacks the necessary fertility to support robust growth. Crops like corn, squash, and pumpkins are nutrient-demanding and should be avoided in the immediate rotation. The soil needs a period of replenishment, ideally through nitrogen-fixing cover crops or soil amendments.
Furthermore, harvesting potatoes involves significant soil disturbance and compaction, as gardeners often dig deeply to retrieve the tubers. This process damages the delicate soil structure, making it a poor environment for the next crop. Deep-rooting vegetables, such as parsnips and carrots, are not a good choice because they require loose, friable soil to develop straight, unblemished roots. The best practice is to allow the soil to rest or plant shallow-rooted, low-demand leafy greens that benefit from the residual fertilizer and help improve the soil structure.