How to Plant Chinese Arrowhead Tubers in a Flooded Bed

What a planting tuber should look like

A seed tuber of Chinese arrowhead is a rounded, slightly flattened knob about the size of a walnut, usually blue-gray to buff, with faint rings around it and a long curved beak at the top. That beak is the sprout, and it is the whole plant in miniature. It can be an inch or two (3 to 5 cm) long already when the tuber comes out of storage, pale and slightly translucent, and it snaps off under very little pressure. A tuber that has lost its beak will sometimes push a weaker second shoot from a side bud, but it will start weeks behind and rarely catches up.

Check each tuber before it goes anywhere near the mud. A sound one is firm all over, with tight skin and no give when you squeeze it between finger and thumb. Reject anything that feels spongy at the base, anything with a sunken dark patch, and anything that has gone light and hollow-feeling from drying out in storage. Arrowhead tubers dehydrate faster than potatoes because their skin is thin, and a shriveled tuber does not rehydrate once it is flooded, it simply sours.

Size matters less than you would expect. A medium tuber with a fat, intact beak beats a large tuber with a broken one every time, because the crop is built from this season's leaves rather than from the food stored in the parent. Keep the tubers in damp sand or wrapped in a damp cloth until the moment you plant, out of the sun, and never let them sit in a warm dry shed for a day waiting for you.

Handle tubers by the body, never by the beak. Carry them in a tray rather than a bucket, where they roll and knock the sprouts off each other.

Wait for the water to warm, not the air

Arrowhead is planted in spring, but the trigger is the temperature of the water in the bed, not a mild afternoon. Plant once the standing water has held above 59F (15C) for about a week and night frosts have finished. Below that the sprout sits still in cold wet mud, which is exactly the condition rot wants, and tubers planted too early are commonly the ones that never come up at all.

There is no advantage in stealing two weeks at the start of the season. The crop is made in late summer and early autumn, when the plant stops making leaves and starts filling tubers, so what matters is a long warm middle and a late finish, not an early start. Planting into 68F (20C) water two weeks late gives a faster, cleaner establishment than planting into 50F (10C) water on time.

If your tubers are ready before the bed is, hold them in a shallow tray with an inch (2.5 cm) of water in a bright frost-free place and change the water every couple of days. They will start to root and the beak will green up, which is fine and even useful, but a sprouted tuber is far more fragile to move, so plant it the day it is ready rather than a week later.

Preparing the mud and setting the tubers

The bed is a watertight container or a lined shallow basin holding six to ten inches (15 to 25 cm) of heavy mineral soil, flooded so that two to four inches (5 to 10 cm) of water stands over the mud. Use plain clay loam or subsoil rather than bagged potting mix: light organic composts float, then rot underwater and turn the mud sour and black. Wet the soil and stir it to the texture of thick porridge, then let it settle for a full day so the surface is smooth and the water above it has cleared before you plant into it.

1. Level and settle the mud first Draw a straight board across the mud to flatten it, then leave it overnight. Soil that is still slumping will swallow the sprouts within a day of planting, and a buried beak rots rather than pushing through.
2. Mark the spacing before you start Push short canes into the mud at the spacing you intend to use, so you do not drift closer together as you work. Once the water is cloudy you cannot see what you have already planted.
3. Press each tuber in, beak up Push the tuber into the soft mud until the body is covered by roughly two inches (5 cm) of soil, with the beak pointing straight up and its tip level with or just above the mud surface. Press with a flat thumb on the shoulder of the tuber, not on the sprout.
4. Close the hole behind it Sweep a little mud over the body so no part of the tuber is exposed to the open water. Exposed tubers get nudged loose, float, and are then found bobbing against the far wall a week later.
5. Let the water clear before you judge it The bed will be opaque for a day or two. Do not go back and poke at tubers you cannot see. When the water clears, count the beaks showing and note any gaps for replanting while there is still time.
6. Top up gently Add water onto a plate or the back of your hand so the stream does not blast craters in the mud and expose the tubers you have just set.

Spacing: leave far more room than the tuber suggests

This is the decision that sets your harvest. Arrowhead does not fatten a tuber underneath itself. It sends out thin white runners horizontally through the mud, sometimes two feet (60 cm) or more, and swells a tuber at the far tip of each one. The crop therefore occupies a circle of mud around the plant, not a lump beneath it, and a bed planted at lettuce spacing simply produces a tangle of runners that cross each other and finish as small tubers jammed against the wall.

Plant at twelve to eighteen inches (30 to 45 cm) each way in a bed, and give a single plant a container no smaller than about twenty inches (50 cm) across. Two plants in a half barrel will yield less between them than one plant would alone. Where you have the room, err wide: an under-filled bed is easy to live with, an overcrowded one cannot be fixed after midsummer.

Contain it now, or regret it later

Every tuber that reaches wet ground becomes a plant. Arrowhead colonises ponds, ditches, boggy corners and wet lawns, and once it is established in open water it is effectively permanent, because hand-pulling leaves the runners and their end tubers behind. Grow it in a sealed, freestanding container or a lined basin standing on hard ground, with the rim several inches clear of the surrounding soil, and never sink that container into a pond, a bog bed or a wet lawn where an overflow can carry a tuber out.

Keep the lining intact and check the rim whenever you top up. Runners travel outward and will climb the inside wall looking for somewhere to go, and a runner that bridges the rim onto wet ground will have set a tuber on the far side before you notice.

Write down the planting date, the water temperature that day, how many tubers went in and exactly where each one sits, with a rough sketch of the bed. In autumn you will be working blind in opaque mud, and next spring you will want to know whether a gap in the crop was a tuber that never came up or one you simply failed to find.