Alliums in Rotation
Part of Crop Rotation
Onions, garlic, leeks, and shallots — the allium family — are among the most nutritionally and medicinally valuable crops available to a rebuilding civilization. They store well, deter pests through volatile sulfur compounds, and occupy a distinct botanical family (Alliaceae / Amaryllidaceae) that integrates cleanly into rotation schedules without competing with most other crop families. Understanding how to grow, position, and store alliums is foundational to year-round food security.
The Allium Family at a Glance
All cultivated alliums share the same family and many of the same growing requirements. They are cool-season crops that prefer moderate temperatures during bulb development, but their specific timings differ.
| Crop | Latin name | Part eaten | Days to maturity | Storage life (dry, cool) |
|---|---|---|---|---|
| Onion (bulbing) | Allium cepa | Bulb | 90–120 days | 6–12 months |
| Garlic | Allium sativum | Bulb (cloves) | 240–270 days (autumn-planted) | 6–10 months |
| Leek | Allium ampeloprasum | Stem/pseudostem | 120–180 days | 2–3 months in ground |
| Shallot | Allium cepa aggregatum | Bulb cluster | 90–120 days | 8–12 months |
| Welsh onion / scallion | Allium fistulosum | Leaf/stem | 60–70 days | Short (fresh use) |
| Chives | Allium schoenoprasum | Leaf | 60 days | Fresh or dried |
Role in Crop Rotation
Botanical Family Separation
The allium family (Alliaceae) is botanically distinct from brassicas (Brassicaceae), legumes (Fabaceae), solanaceae (nightshades), and cucurbits (Cucurbitaceae). This makes alliums an excellent rotation partner — they share no soil-borne diseases with these major families.
Allium-specific pathogens include:
- White rot (Sclerotium cepivorum): persists in soil for 20+ years; most damaging allium disease
- Allium leaf miner (Phytomyza gymnostoma): adult fly damage
- Onion fly (Delia antiqua): larval root damage
- Downy mildew (Peronospora destructor): airborne but worsened by dense planting
To avoid these, do not grow any allium species on the same plot more often than once every three years, and once every four or five years where white rot has been observed.
Recommended Rotation Position
Alliums make few demands on nitrogen (they are light feeders) but prefer a fine, well-drained seedbed with moderate fertility. The ideal rotation slot is after a heavy-feeding crop that received compost:
Year 1: Heavy feeder (cereal, maize, squash) — receives full compost amendment
Year 2: Alliums — benefit from residual fertility, minimal additional input
Year 3: Legume — rebuilds nitrogen after the sequence
Year 4: Root crop or brassica
Never follow legumes directly with alliums. The high nitrogen from legume residue produces excessive leaf growth in alliums at the expense of bulb development.
Alliums After Potatoes
Potatoes and alliums make a practical pairing. Potatoes break up compacted soil with their root growth, leaving a friable tilth ideal for onion sets. Potatoes also suppress some soil-borne weeds. Plant alliums the year after potatoes, adding only a light dressing of aged compost (1–2 kg/m²).
Growing Alliums
Soil Preparation
All alliums require:
- pH 6.0–7.0 (test with indicator; lime if below 6.0)
- Good drainage — they rot in waterlogged conditions
- Fine, crumb-structure surface (rake thoroughly)
- Low fresh organic matter — avoid fresh manure
Incorporate aged compost at 1.5–2 kg/m² at least four weeks before planting. Avoid fresh manure; it promotes neck rot and soft bulbs.
Planting Methods
Onions from sets (small bulbs): Push sets 2 cm deep, 10 cm apart, in rows 25 cm apart. Sets planted in autumn (in mild climates) or early spring give the longest growing season. Cover lightly with soil; birds pull them up — weigh down temporarily with fleece or netting.
Garlic from cloves: Break bulbs into individual cloves. Plant pointy end up, 5 cm deep, 15 cm apart, rows 30 cm apart. Autumn planting (8–12 weeks before hard frost) is essential for most varieties; garlic needs a cold period (vernalization) to form multiple cloves. Spring-planted garlic often produces single-clove rounds.
Leeks from transplants: Sow seed in trays or a nursery bed, thin to 2 cm apart. Transplant at pencil-thickness (12–16 weeks after sowing). Use a dibber to make holes 15 cm deep and 15 cm apart. Drop transplants in without backfilling — rain and watering will settle soil around roots. Deep holes blanch the lower stem, producing the white, tender shank.
Shallots: Plant individual bulbs as with onion sets, 15–20 cm apart. Each bulb multiplies into a cluster of 6–12 daughter bulbs by harvest.
Water Requirements
Alliums have shallow roots and are drought-sensitive during bulb formation. Water at 15–20 mm per week during active growth. Reduce or stop watering as tops begin to fall — this triggers skin hardening essential for storage. Excess moisture at maturity causes soft necks and rot.
| Growth stage | Water need |
|---|---|
| Germination / establishment | Consistent moisture |
| Active leaf growth | 15–20 mm/week |
| Bulbing (tops still upright) | 10–15 mm/week |
| Maturation (tops falling) | Stop or greatly reduce |
Weed Management
Alliums have narrow, upright leaves that cast minimal shade. Weeds compete strongly, especially in early growth. Hoe shallowly and frequently — allium roots are near the surface and deep cultivation damages them. Hand-weed within 5 cm of plants. A 3–5 cm mulch of straw or leaf mould between rows reduces weed pressure but keep it away from bulbs themselves to prevent rot.
Harvesting
Onions and shallots: Harvest when 50–75% of tops have fallen naturally. Do not force tops down. Ease bulbs from the ground with a fork, avoiding bruising. Lay out in a single layer to dry in sun and wind for 1–3 weeks. In wet climates, dry under cover with good airflow.
Garlic: Harvest when lower leaves have yellowed (3–4 leaves still green). Dig rather than pull to avoid breaking the neck. Dry as for onions. Softneck varieties store longer than hardneck; braid softneck types for hanging storage.
Leeks: Harvest by digging with a fork at any size from pencil thickness to full maturity. They can remain in the ground through mild frosts. In hard winters, mulch the row with 10 cm of straw to extend in-ground storage.
Cure Before Storage
Uncured alliums rot within weeks. The curing process seals the neck and dries the outer skins into a protective papery layer. Full curing requires 2–4 weeks at 25–30°C with good airflow, or 4–6 weeks at 15–20°C. Do not rush this — it determines storage life.
Storage
Once cured, store alliums in the following conditions:
| Condition | Target |
|---|---|
| Temperature | 0–4°C (cold storage) or 15–20°C (warm dry) — avoid 4–15°C (promotes sprouting) |
| Humidity | 60–70% relative humidity |
| Airflow | Mesh bags, slatted crates, or braided strings — never airtight |
| Light | Dark — light triggers sprouting |
The optimal choice is either cold storage (near freezing, high humidity) or warm dry storage (above 15°C, low humidity). The middle range (4–15°C) causes rapid sprouting and is the worst possible environment.
Inspect stored alliums every two weeks. Remove any showing soft spots, mould, or sprouting. Use imperfect bulbs immediately for fresh eating or drying.
Drying for Long-Term Storage
Slice onions and garlic thinly (3–5 mm). Dry at 50–60°C in a simple solar dryer or low oven until brittle (8–12 hours). Store in sealed containers away from moisture. Dried alliums last 1–2 years and rehydrate well in cooking. Powder by crushing dried slices — a small jar of onion or garlic powder provides months of flavour.
Medicinal and Pest-Deterrent Uses
Alliums contain allicin and related sulfur compounds with documented antimicrobial properties. Crushed garlic applied to wounds reduces infection risk in the absence of antibiotics. Garlic tea (several crushed cloves steeped in hot water) has been used traditionally against respiratory and digestive infections.
In the garden, interplanting alliums with vulnerable crops deters some pests through volatile sulfur compounds. Particularly effective pairings:
- Garlic or chives near roses and stone fruit (deters aphids)
- Leeks or onions near carrots (mutual pest deterrence: allium fly vs. carrot fly)
- Garlic planted around the edge of a brassica bed (deters cabbage white butterfly to a limited degree)
Do Not Interplant Alliums with Legumes
Alliums suppress the growth of beans, peas, and other legumes. Keep them separated in the field, in the rotation plan, and in storage areas where volatile compounds can affect germination of nearby legume seeds.
Saving Allium Seed
Most alliums are biennial — they flower in their second year. To save seed:
- Select the best bulbs at harvest and store through winter.
- Replant in spring; plants will bolt (flower) in their second growing season.
- Allow flower heads (umbels) to mature and dry on the plant.
- Cut when seeds begin to blacken; complete drying in paper bags.
- Thresh by rubbing; winnow in breeze.
Onion seed viability drops sharply — use within 1–2 years. Garlic is propagated vegetatively (cloves), not from true seed; select the largest, healthiest bulbs each year for replanting.
Alliums in Rotation Summary
Alliums occupy a distinct botanical family, making them ideal rotation partners that share no pathogens with brassicas, legumes, solanaceae, or cucurbits. Place them after heavy feeders that received full compost, not after legumes. Rotate the allium plot every 3–4 years minimum to avoid white rot buildup. Garlic requires autumn planting and vernalization; onions and shallots can be planted from sets in spring; leeks are transplanted from nursery beds. Cure all bulb alliums for 2–4 weeks before storage. Store in either cold (0–4°C) or warm dry (15–20°C) conditions — never in the mid-range where sprouting accelerates. Reserve the best bulbs and cloves each year for replanting stock.