“Replant disease” occurs when a plant is introduced close to where one of the same genus has recently grown. Essentially, the previous occupant depleted all the nutrients, caused a build-up of pests and left residues of allelopathic compounds in the space, making it inhospitable for the same, or a similar plant to occupy the same space the following year. In commercial agriculture, therefore, crop rotation is rigorously adhered to, but it’s important to apply the basic principles to gardening on a smaller scale to ensure healthy crops.
Plants are grouped into families, members of which can generally be planted together the same year, but you should avid planting members in the same space in consecutive years. For example, you can plant cabbages and broccoli in the same raised bed one year, but do not plant them in that same raised bed the following year as the ground will have been depleted of nitrogen, and club root disease may have built up. Instead, you plant Alliums, which don’t require nitrogen, followed by potatoes, whose dense foliage will suppress weeds, and whose tubers “dig” the soil for you, aerating the soil. Next, plant nitrogen-fixing crops such as beans in that space, to re-nitrogenate the soil for the next round of brassicas. Rotation essentially uses the principle of each crop having a beneficial impact on the crop that will follow it.
In a small garden rotation is less relevant because pests can travel more easily than they would in a large space, but its worth thinking about when using the same containers year-on-year. (In other words, don’t plant potatoes in the same pot for two consecutive years – use the container for another crop first to give the soil time to recuperate).
The families to avoid planting members of successionally are:
Curcubitacae (courgette, squash, cucumber, melon)
Leguminaceae (peas, beans)
Allium (onions, garlic, leeks, chives, shallots)
Solanaceae (potato, tomato, aubergine, pepper, chilli)
Umbelliferae (carrots, celery, celeriac, parsnips, fennel, parsley)
Chenopodiaceae (chard, beetroot, spinach, quinoa)
Asteraceae (lettuce, chicory, jerusalem artichokes, sunflowers)
Brassicaceae (broccoli, brussel sprouts, cabbage, cauliflower, kale, swede, koh rabi, kale, turnip, radish)