Harvesting onions and garlic, both members of the Allium family, provides an opportunity for succession planting. This practice allows gardeners to maximize yield from a single plot of soil by immediately replacing the harvested crop with another that can mature before the end of the growing season. However, simply dropping new seeds or transplants into the vacated space is not advisable. A strategic approach based on crop rotation is necessary to maintain soil health and productivity, accounting for the specific changes the alliums have left behind in the soil.
Understanding Allium Soil Impact
Crop rotation after harvesting onions or garlic is essential due to the risk of disease and pest carryover. Fungal pathogens specific to the Allium family, such as Fusarium basal plate rot or pink root, can persist in the soil for several years. Planting another allium immediately in the same spot would provide a host for these pathogens, guaranteeing a cycle of infection and poor yield. Pests like onion maggots and bulb mites leave behind pupae or eggs, which can quickly infest a subsequent crop if it is from the same family.
Although alliums are often described as light feeders, they have particular nutrient requirements that can deplete specific micronutrients. Onions are sensitive to deficiencies in manganese, molybdenum, and zinc. Furthermore, the decomposition of allium roots and debris can release sulfur compounds that may temporarily affect the germination of some seeds. Therefore, the rotational crop should have different nutritional needs and no susceptibility to allium-specific diseases.
Ideal Follow-Up Crops
The best succession crops after onions and garlic are those that either replenish the soil or do not compete for the same nutrients and are resistant to allium pathogens. Legumes, such as bush beans and summer peas, are excellent choices because they fix atmospheric nitrogen into the soil through a symbiotic relationship with rhizobia bacteria. This nitrogen enrichment naturally prepares the bed for a future heavy-feeding crop while legumes themselves require little nitrogen to thrive.
Heavy-feeding crops from the Brassica family, such as kale, cabbage, or broccoli, also perform well, especially for a fall harvest. These plants benefit from the residual nitrogen and organic matter left by the previous crop. Their deep, fibrous root systems explore different soil layers than the shallow allium roots, helping to break up the soil structure and utilize nutrients more broadly.
Fast-maturing root and leafy green vegetables are suitable for a quick second harvest in mid-to-late summer. Carrots, radishes, lettuce, and spinach have different disease profiles and can be harvested before the first hard frost. These quick-turnaround crops are ideal for gardeners with shorter growing seasons, as they quickly fill the space and provide a yield.
Crops to Strictly Avoid
The most important rule of rotation is to strictly avoid planting any other member of the Allium family in the same location. The fungal spores from diseases like white rot or Fusarium basal rot are capable of surviving in the soil for years and will rapidly infect a new allium crop. This includes:
- Chives
- Leeks
- Shallots
- Ornamental alliums
The presence of soilborne pests, such as onion maggots and bulb mites, makes the bed a hazardous environment for any related plant species. Replanting alliums would ensure the pests have a continuous food source, encouraging their population to grow and spread to other parts of the garden. A rotation period of at least three years away from all alliums is recommended to effectively starve out these persistent pests and pathogens.
Preparing the Soil for Succession Planting
Proper preparation of the bed immediately after the allium harvest is necessary for a healthy succession crop. The first step is the thorough removal of all plant debris, including leaves, stems, and any small bulb remnants. This material can harbor fungal spores and pest eggs, and should be disposed of outside the garden, not added to the compost pile.
Once the bed is clear, the soil should be lightly disturbed with a fork or rake to prepare a seedbed. This is the optimal time to incorporate soil amendments, such as finished compost, to replenish organic matter and feed the soil microbes. If planting a heavy feeder like a brassica, a balanced organic fertilizer can be lightly worked into the topsoil.
If legumes are the follow-up crop, reduce or eliminate the addition of nitrogen fertilizer, as excess nitrogen will discourage the plants from forming the beneficial, nitrogen-fixing root nodules. For most summer-to-fall succession planting, mid-summer timing is ideal. This allows the new crops to benefit from the warm soil and existing infrastructure, such as irrigation lines.