How to Plant Oxalis Bulbs: Pot Size, Depth and Why Not a Border

Decide between a pot and open ground before you plant anything

Oxalis is a genus of more than eight hundred species, and it contains both the purple-leaved plant sold as a houseplant and several of the most persistent garden weeds in the temperate world. They spread the same way: each mother bulb builds a cluster of tiny detachable bulbils around its base, and every one of those bulbils that stays in the soil becomes a plant. Digging a weedy species out shatters the cluster and multiplies it, which is why gardeners who have fought one of them never plant another on trust.

The ornamental kinds grown for their triangular purple leaves or their four-lobed shamrock foliage are far better behaved than the notorious weeds, but they still increase by bulbils, and in a mild climate with free-draining soil they will drift outward through a border a few inches a year and turn up in the gravel path. If you do not know with certainty which species you have, treat it as if it spreads and grow it in a container. That is not a limitation. This is a plant that looks best in a wide shallow pot on its own anyway, where the leaves can mound up over the rim, and a pot lets you lift the whole thing out of the weather and out of its dormancy without disturbing anything else.

The only situation where open ground is straightforwardly sensible is a bed you are content to hand over entirely, with a hard edge such as a path or a wall on every side, in a climate cold enough that winter kills any stray bulbil outside the mulched planting area. If you are planting into a mixed border with small perennials in it, use a container sunk to its rim instead and lift it at the end of the growing season.

What you are actually handling: scaly bulbs that break apart on purpose

Depending on the species, oxalis is sold as small scaly bulbs, as brittle segmented rhizomes that look like pale fir cones a quarter of an inch (6 mm) across, or as fat little tubers. They all behave the same in the hand: they are dry, they look far too small and dead to become anything, and they snap apart easily. Do not fight that. A rhizome that breaks into three pieces gives you three plants, and every piece with a scale on it will grow.

There is no top and bottom you need to worry about. Oxalis bulbs have contractile roots that pull the growing point to the depth it wants, so a bulb planted sideways or upside down straightens itself out in the first fortnight. What does matter is that they are firm. Discard anything that is soft, hollow, or has a dark wet patch, because that is rot and it will spread through a pot of stored bulbs.

Plant them as soon as you can after they arrive. These are not tulips and they do not store well for long once lifted; a bulb kept dry in a paper bag for several months loses vigor and comes up thin. If you have to hold them, keep them in barely damp coir at around 50 to 60F (10 to 16C) in the dark and check them monthly.

Pot, compost and planting depth

Oxalis has a small root system relative to the amount of leaf it carries, so it is planted densely and in a container that is wide rather than deep. A six inch (15 cm) pot with six to eight bulbs gives a full dome of foliage in about six weeks. The same six bulbs spread through a twelve inch (30 cm) pot look sparse for an entire season and the extra volume of wet compost around them raises the risk of rot. When in doubt, use a smaller pot and more bulbs.

The compost has to drain fast. A standard peat-free multipurpose mix on its own stays wet for too long around a dormant or a newly planted bulb. Cut it by about a third with horticultural grit, perlite or coarse bark, so that water runs out of the base within a few seconds of watering. Any pot must have drainage holes; oxalis in a sealed cachepot with standing water in the bottom is a rotted bulb waiting to happen.

Planting, step by step

The whole job takes ten minutes and the only skill in it is restraint with the watering can afterwards. A newly planted bulb has no roots and no leaves, so it cannot use water, and compost kept soaking through those first weeks is the commonest way to lose a batch.

1. Fill the pot to within two inches of the rim Use the gritty mix and firm it lightly with your fingertips rather than pressing it hard. You want it settled, not compacted, because these small bulbs push through it with very little force.
2. Space the bulbs across the surface Set them about an inch (2.5 cm) apart in a rough grid across the whole surface, not in a ring round the edge. Orientation does not matter. If some have already begun to sprout a pale shoot, set those the right way up and handle them gently.
3. Cover with one inch of compost Add enough mix to give an inch (2.5 cm) over the top of the bulbs and level it off. Leave half an inch (1 cm) of clear space below the rim so you have somewhere to pour water.
4. Water once, then stop Water thoroughly until it runs out of the base, let it drain completely, and then do not water again until you see the first shoots break the surface. Depending on temperature that is two to four weeks. Bone dry is safer than damp at this stage.
5. Put it somewhere bright and mild A spot at 60 to 70F (15 to 21C) with good light but no fierce direct sun. A windowsill out of the midday beam, a greenhouse bench, or a sheltered outdoor table once nights are reliably above 50F (10C).
6. Start normal watering when the leaves unfold Once shoots are up and the first leaves have opened, water whenever the top inch (2.5 cm) of compost is dry. Growth accelerates fast from this point and a pot in full leaf uses water quickly.

The first season, and the two dates worth writing down

Expect a full pot within six to eight weeks of the shoots appearing, and the first flowers a few weeks after that in most of the ornamental kinds. Do not feed until the pot is in full leaf; a bulb living on its own reserves has no use for fertilizer and the salts sit in the compost. Once it is growing hard, a balanced liquid feed at half the label strength every three to four weeks is plenty.

Write down the date you planted and the date the first leaf opened. Those two dates give you the lead time for this plant in your conditions, and lead time is the number you need every time you restart it after dormancy, which for most people is at least once a year. Add how many bulbs went into which pot. When you tip it out at the end of the cycle and find four times as many bulbs as you planted, that original count tells you how hard to thin.

One further note worth making, especially if you are planting outdoors: mark exactly where the planting area ends. Oxalis bulbils travel, and two years later, when you find leaves eighteen inches (45 cm) beyond where you thought you planted, a recorded boundary is the difference between knowing you have a spreader and guessing.

If you are planting into open ground in a mild climate, line the planting hole with a bottomless container sunk to its rim, or plant into a sunken pot you can lift whole. Retrofitting containment once bulbils are loose in a border is close to impossible.