How to Plant Calla Lily Rhizomes Without Rotting Them

Work out which kind you are holding first

Everything about planting a calla depends on which of two very different plants you have, and they are sold under the same name. The tall white calla is a marsh plant from a summer-rainfall region that grows at the edge of water, keeps its leaves through mild winters and can stand in three to six inches (8-15 cm) of water all season without complaint. The smaller colored callas, the yellows, pinks, oranges and near-blacks, come from drier ground, die back completely every year and rot within days if given the same treatment.

You can usually tell them apart from the storage organ itself. The white kind produces a thick fleshy rhizome, often already carrying pale roots and sometimes green shoot tips even out of the ground, and it feels heavy and moist. The colored kinds are sold as flattened dry tubers, brown and corky on the outside, with several distinct dimpled growing eyes on the upper surface and no live roots at all. A dry tuber is not a dead tuber; that is how it is supposed to arrive.

The leaves confirm it later. The white kind carries broad arrow-shaped leaves in plain glossy green. The colored kinds nearly always have narrower leaves speckled with translucent white flecks, which is the quickest field mark once anything is growing.

From here on the two are treated separately, because there is no single planting instruction that suits both.

Planting the white kind: wet ground, and plenty of room

The white calla will grow in an ordinary border if it is watered generously, but it is a different plant altogether at the edge of a pond or in a bed that stays damp. Choose the wettest place you have. Ditch bottoms, the low corner of a lawn, the overflow side of a water butt and the shallow margin of a pond are all ideal, and the plant will reach three feet (90 cm) or more with leaves twice the size of a border-grown clump.

Plant the rhizome horizontally with the growing point upward, three to four inches (8-10 cm) below the surface, spacing clumps at least eighteen inches (45 cm) apart. That spacing looks absurd with a single rhizome in your hand and is barely enough by the second season. In a pond margin, use a mesh aquatic basket lined with hessian, filled with heavy loam rather than a peat-based potting mix, topped with a layer of gravel to keep the soil in place, and stand the basket so the crown sits under three to six inches (8-15 cm) of water.

In open ground it wants full sun where summers are cool and half a day of shade where they are hot, and it is greedy. A generous quantity of compost or rotted manure dug into the planting area is used rather than wasted, which is the opposite of the advice for most bulbs. Water it in heavily and then simply never let it dry out.

Where winters drop well below freezing, treat the white calla either as a container plant that comes into a frost-free place or as a plant to lift. In the ground it survives cold that mild-winter gardens see, particularly under six inches (15 cm) of mulch, but a rhizome frozen solid in saturated soil is lost.

Every part of the calla is poisonous and the sap irritates skin. It contains needle-shaped calcium oxalate crystals that cause immediate burning and swelling in the mouth if chewed. Wear gloves when cutting or dividing rhizomes, wash your hands afterwards, and keep the cut pieces away from children and pets.

Planting the colored kinds: warm soil and an unbroken skin

The colored callas fail at planting far more often than they fail later, and almost always for the same two reasons: they went into ground that was too cold and too wet, or the tuber was damaged on the way in. The bacterium behind soft rot cannot get through intact corky skin, but it walks straight into a snapped edge, a bruise or a knife cut, and in cool wet soil a tuber can be liquid inside a fortnight while showing nothing at the surface.

So wait. Soil at 60F (16C) is the threshold, measured a few inches down in the morning rather than on a warm afternoon, and there is nothing to gain from planting into anything colder. Tubers held back a fortnight in a warm dry room catch up almost entirely, and they root immediately once they go in.

Handle them as if they bruise, because they do. Do not drop them into the hole, do not press them down hard with a thumb, and do not tidy them up with a knife. If you must divide a large tuber, do it a few days before planting with a clean blade, leaving at least two eyes on each piece, and leave the cut faces open to the air in a warm dry place until they harden into a dry corky callus. Planting a fresh wet cut face is the single most reliable way to lose the tuber.

Set them three to four inches (8-10 cm) deep with the dimpled eyes facing up, eight to twelve inches (20-30 cm) apart, in soil that has been improved with compost but drains freely. On heavy clay, plant into a raised bed or a container instead, because clay that stays wet in a cool early summer will take them however carefully they were handled.

1. Check every tuber before it goes near soil A sound tuber is firm all over and dry to the touch. Squeeze it gently: any softness, any weeping, any sour smell, any patch that gives under the thumb means it is already rotting inside. Throw those away rather than planting them at the end of the row to see what happens, because they contaminate the soil around them.
2. Wait for 60F (16C) soil Push a soil thermometer three inches (8 cm) down and read it in the morning for several days running. The tuber does nothing below this temperature except sit in cold moisture losing its resistance to rot, and a planting two weeks later will overtake an early one.
3. Plant eyes upward and shallow rather than deep Three to four inches (8-10 cm) of soil over the top is right. Eyes are the small dimples or raised buds on the flatter upper face. If you genuinely cannot tell which way up it goes, plant it on its side; the shoot will find its own way up and this is far safer than guessing wrong and planting a tuber upside down at depth.
4. Do not water it in This is the reverse of normal practice and it matters. Water the ground the day before if it is dust dry, then plant and leave it alone. The tuber has all the moisture it needs to make its first roots, and standing water round a freshly planted tuber in cool soil is how soft rot starts.
5. Start watering only when shoots appear Expect two to four weeks to emergence in warm soil. Once the first spear of leaf is through, begin watering normally. Until then, keep the bed on the dry side of moist and resist the temptation to check by digging.

Containers, which suit the colored kinds particularly well

The colored callas are easier in pots than in the ground for most gardens, because a pot lets you control both the temperature and the drainage that they are so fussy about. Use a container at least eight inches (20 cm) across with real drainage holes, filled with a free-draining potting mix, and set one to three tubers per pot depending on size, three inches (8 cm) deep.

Started under cover in a frost-free place a few weeks before the ground outside is warm enough, potted tubers are already in leaf when they go out, which brings the flowers forward by a fortnight or more and skips the riskiest part of the season entirely. Move them outside only when nights stay above 50F (10C).

The white calla also grows well in a large container, but its requirement is the opposite one: a pot with no drainage hole at all, or a pot standing in a saucer kept permanently full, is exactly right for it. A twelve inch (30 cm) pot standing in three inches (8 cm) of water will support a full-sized flowering clump all summer.

What to write down at planting

Record which kind went where, the date each tuber or rhizome was planted, the soil temperature you planted the colored ones into and the date the first shoot broke the surface. That last figure is the most useful of all: emergence time tells you whether the soil was warm enough, and a batch that takes six weeks to appear was planted too early whatever the calendar said.

Note the position of each planting precisely, ideally with a marker that survives the winter. Dormant colored callas leave nothing above ground for half the year and are easily speared with a fork while planting something else. Recording how many tubers went in also gives you the number to compare against at lifting, which is the honest measure of whether the season built the tubers up or wore them down.