Chinese Arrowhead Care: Thinning Shoots, Feeding and Managing Runners

Reading the plant above the water

Everything that matters is buried in opaque mud, so the leaves are the only progress report you get. The first growth from a planted tuber is often narrow, strap-like and lying on or under the water. Those juvenile leaves mean the tuber is alive and rooting and nothing more. What you are waiting for is the true arrow-shaped blade, held clear of the water on a stiff spongy stalk, with two backward-pointing lobes at its base. Once several of those are standing, the plant is feeding itself and the runners have begun to travel.

Height and count together tell you how the season is going. By midsummer a well-grown plant carries a dozen or more arrow leaves standing one to two feet (30 to 60 cm) above the water, with the blades overlapping into continuous cover. A plant that reaches midsummer with only a few short leaves is too cold, too shaded, too crowded, or was planted from a tuber whose sprout was damaged, and it will not make much of a crop that year.

Late in the season the plant stops sending up new leaves and the existing ones start to yellow from the outside in. That is not a problem to fix. It is the signal that the runners have stopped traveling and the tubers at their tips are filling, and it is the point at which feeding stops for the year.

Thin the shoots or the crop stays small

A single tuber usually throws more than one shoot. Left alone, three or four crowns grow up from the same spot, compete with each other for the same patch of mud, and share out the same amount of light and food across three or four sets of runners. The result is a large number of tubers the size of a thumbnail instead of a smaller number worth cooking.

Reduce each planted tuber to one strong crown, or at most two in a large bed. Do it when the shoots are a few inches (about 10 cm) tall and you can still see which is strongest, and do it in one go rather than gradually. Do not cut the surplus at the mud surface: pull it, with a finger pushed down beside the shoot to hold the parent tuber in place, so the whole shoot comes away rather than leaving a stub that re-sprouts twice more.

Weeds work the same way and need the same treatment. Duckweed and floating pondweeds arrive on their own and thicken into a mat that shades the water and hides the level mark. Skim them off whenever you top up. Rooted weeds in the mud should be pulled young, because pulling a large one late tears through the arrowhead runners that have already crossed under it.

Thinning feels wasteful when the bed looks sparse in early summer. It is the difference between a crop you can cook and a crop you can only admire.

Feeding: buried, moderate and stopped early

Arrowhead is a hungry crop and the mud in a container runs out by midsummer, but almost every feeding mistake comes from putting fertilizer in the water instead of in the soil. Anything scattered on the surface or dissolved in the water feeds algae, turns the bed green and does nothing for the plant, and heavy surface dressings sour the mud. Everything goes underneath, pushed down with a finger or a length of dowel.

1. Do not feed at planting A newly planted tuber has no roots to take anything up, and fertilizer sitting next to it in warm mud simply encourages rot around the sprout. Wait for the first true arrow leaf.
2. First feed when the arrow leaves stand Push aquatic plant tablets three to four inches (8 to 10 cm) into the mud, roughly halfway between the plant and the edge of the bed, so the runners grow into the feed rather than away from it.
3. Repeat every three to four weeks Through early and midsummer, use about one tablet per gallon (4 L) of soil each time, or the rate given on the pack. Close each hole behind the tablet so nothing leaches out into the open water.
4. Favor potassium late, not nitrogen Heavy nitrogen in late summer pushes soft new leaves at exactly the moment the plant should be filling tubers. If you feed late at all, use a low-nitrogen aquatic feed.
5. Stop about six weeks before the tops die back Once the plant has sent up its last new leaf, feeding is finished. Late feeding delays tuber filling and produces soft tubers that keep badly in storage.
6. Let the leaves set the rate Pale yellow-green new leaves with darker veins mean it is hungry. Very dark leaves with scorched brown margins mean too much feed too near the surface: overflow the bed with clean water and skip the next round.

Leaves, runners and keeping the bed in bounds

Do not cut leaf stalks below the waterline. They are air tubes running down into the runners and roots, and a stalk cut underwater floods, carrying water into the tissue it serves and rotting it. Remove only leaves that are already yellow or brown and finished, cut them above the surface, leave a hand's width of stalk standing, and let that stub dry and collapse on its own. If a storm snaps a stalk below the surface, cut the remaining stub off above the water the same day rather than leaving an open flooded funnel.

Resist tidying healthy leaves. Every arrow leaf removed in midsummer is food the developing tubers do not get, and the crop is built in the last third of the season on exactly those leaves.

From midsummer, check the rim of the bed every time you top up. Runners travel outward and will climb the inside wall looking for open ground, and a runner that reaches over the edge onto wet soil sets a tuber there that you will not find until it comes up as a plant next year. Push any runner you find at the edge gently back down into the mud. Never let the bed overflow into a pond, a ditch or a bog garden, because arrowhead that reaches open water is not removable by hand.

Keep a short running note through the season: the date the first arrow leaf stood clear, how many shoots you thinned to, the date of each feed, and the leaf count and height at midsummer. Set beside what you dig in autumn, those few numbers tell you next spring whether to thin harder, feed sooner, or give each plant more room.