Gadenin日本語Get the appWhy the bed stays flooded all season
Chinese arrowhead grows in mud that has no oxygen in it. The plant handles that by breathing through its leaf stalks, which are spongy tubes that carry air from the leaf blade down into the roots and runners. That system only works if the mud stays waterlogged and the soil structure stays soft and closed. As soon as the surface drains and air gets into the mud, the mud sets hard, the runners stop traveling, and the plant switches from making tubers to simply surviving.
So there is no watering schedule in the ordinary sense. You are maintaining a level, the way you would maintain the level in a pond, and the job for most of the season is topping up what evaporates rather than deciding whether the plant is thirsty. A bed that never shows bare mud between spring and autumn has been watered correctly, whatever the weather did.
Standing water does a second job that is easy to overlook: it buffers temperature. Several inches of water over the mud holds warmth overnight and keeps the root zone from swinging with every cold night in late spring, which is why a shallow bed and a deep one behave very differently in an unsettled season.
The level changes through the season
Start shallow and go deeper as the plant grows. Shallow water warms fast in spring, which is exactly what a newly planted tuber needs, and the young sprout must be able to reach the surface without drowning. Once the leaves are standing well clear, a deeper body of water is safer because it takes longer to evaporate away and holds a steadier temperature. Then, at the very end, you drop the level again so the tops can finish and dry off.
Raise the level in steps, not in one go. A young plant whose leaves are suddenly submerged by four inches (10 cm) of fresh water will lose those leaves, and it has no reserves to replace them with in early summer.
| Stage | Water over the mud | Why |
|---|---|---|
| Planting to first standing leaf | 2 in (5 cm) | Warms fastest. The sprout must reach air quickly. |
| Early growth, leaves under a foot (30 cm) | 2 to 3 in (5 to 8 cm) | Keeps warmth while leaves are still short. |
| Main season, full leaf cover | 4 to 6 in (10 to 15 cm) | Buffers heat and evaporation. Runners are traveling. |
| Heatwave, over 90F (32C) | 6 in (15 cm) plus | Extra depth stops the shallow mud from cooking. |
| Tubers filling, leaves yellowing | 2 to 3 in (5 to 8 cm) | Lets the mud cool and the plant finish. |
| Tops dead, before lifting | Drained | Mud firm enough to work by hand. |
Topping up without wrecking the bed
In high summer a full bed of arrowhead can lose an inch (2.5 cm) of water a day. Broad leaves standing above an open water surface transpire hard and there is nothing shading the water, so expect to check every day in hot weather and every second or third day in mild weather. Get in the habit of checking at the same time each day, because a level that looks fine in the cool of the morning can be two inches (5 cm) down by evening.
- Mark the fill line Scratch or tape a permanent mark on the inside wall at the level you want for the current stage. Judging by eye across a wide bed is unreliable, and a mark turns the daily check into a two-second glance.
- Break the flow Run the hose or can onto a plate, a stone or the back of your hand. A direct stream digs a crater in the mud, exposes runners and clouds the bed for two days.
- Use water that has stood Where you can, top up from a butt or from a can left in the sun. A slug of cold mains water on a hot afternoon drops the bed temperature sharply and checks growth for several days.
- Top up little and often Adding an inch (2.5 cm) daily is far better than four inches (10 cm) once a week. Big swings in level leave a tidemark of dried mud around the edge, and that dried rim cracks and lets air into the soil.
- Never let it go dry, not even once A single day of exposed mud sets the surface, kills the fine feeding roots and stops the runners. The plant looks fine above the water for a week and then simply stops making tubers.
- Refresh rather than replace If the water goes foul, let clean water overflow the rim slowly for a few minutes instead of draining and refilling. Draining exposes the mud, which is the very thing you are trying to avoid.
Reading the water itself
The water tells you about the mud underneath it. Clear water with a light film of green algae is normal and harmless; algae competes for a little nutrient but does nothing to the crop, and it can be lifted off by hand or with a net if it thickens into a mat. Cloudy brown water after a top-up is just disturbed silt and settles in a day.
What you do not want is water with an oily rainbow film, a smell of rotten eggs, or bubbles rising when you push a finger into the mud. All three mean the soil has gone sour, usually because there is too much undecomposed organic matter in it or because a heavy feed was scattered on the surface instead of buried. Flush the bed by letting clean water overflow slowly, stop feeding, and remove any leaf litter lying on the mud.
Note the depth you are holding at each stage and roughly how often you top up through the hottest weeks. Comparing that record with what you dig in autumn is the quickest way to work out whether the bed was too shallow for your climate, and it tells you next spring whether the bed needs to be deeper, shaded at the edge, or simply bigger.
If you go away in summer, raise the level to the top of the bed before you leave and ask someone to top it up. Arrowhead survives a week of neglect only if it starts that week full.