Advice
RHS Help & Advice
Planting onion sets, shallots and garlic
Onions can be grown from seed or from sets (baby bulbs). Sets are very easy to plant and grow, and escape many of the problems to which seedling onions are prone.
In this guide, we show how to plant onion sets, shallot sets and garlic (which can be grown from individual cloves).
Plant between February and April, as soon as the soil is sufficiently dry and warm; in practice this is usually late winter or early spring for sandy soils, and mid-spring for clay-based soils. Heat-treated sets (which have had their flowering potential suppressed, so are bolt resistant) should not be planted before late March or April.
When selecting untreated sets, choose smaller bulbs over large ones, as smaller bulbs may be more resistant to bolting (they are less mature, so have less well developed flower buds).
Start by raking over prepared soil to produce a fine level tilth (surface).
Onion and shallot sets are planted into a shallow drill (groove) in the soil, created with a string line and a draw hoe (left), or by laying a plank across the bed and running a trowel along its edge (right). The drill should be about the same depth as the set, or a bit less for some of the longer sets (many shallots, for example).
Remove any loose papery skins before planting the sets.
Push the sets into the soil at the base of the drill, with their pointed tips upwards. Spacing can be anything from 2.5cm (1in) to 10cm (4in) apart, depending on the size of bulbs required. Closer spacing results in large numbers of small bulbs, whereas wider spacing results in a smaller number of large bulbs; 5-7.5cm (2-3in) usually works well.
Fill in the drill with soil by running the edge of the rake along its edge to draw soil over the sets (left), or use the trowel in a similar fashion (centre). Use the trowel to firm in the sets (right).
When they are planted, the tips at least should still be protruding from the soil surface.
Garlic cloves are planted into individual holes 10cm (4in) deep, which are then backfilled with the trowel. The cloves should not protrude from the soil surface, and are placed in the holes with their pointed tips upwards. A light soil and sunny site are ideal. Heavy soils should be lightened first, by mixing the top layer with sharp sand before planting.
In the spring there is rarely need to water newly planted sets. But in dry spells later on in the summer and autumn, new plantings of overwintering onion sets should be watered in after planting. In very dry weather, it may be a good idea to water the drill before planting.
As garlic needs some cold weather to initiate flowering, only certain cultivars are recommended for spring planting. Try ‘Sultop’ or ‘Spring White’.
Maya Albert