Gadenin日本語Get the appAphids on the standing leaves
Aphids are the pest you will actually meet, usually black or dark green, massed on the newest rolled leaves and on the upper inch or two of the leaf stems where the tissue is soft. They arrive in warm settled weather in early summer and build up fast, because the tall stems make an obvious landing site above open water.
Catch them at the point where a single new leaf has a dark collar around its stem. At that stage you can wipe them off with a wet cloth held around the stem, or knock them into the water with a jet from a hose, where they drown. Repeat every couple of days for a week and the colony usually does not re-establish. If the tub holds small fish, they will clear anything knocked into the water for you.
Do not reach for insecticide over standing water. Anything you spray runs straight off the waxy leaf surface into a body of water you cannot flush, and it stays there for the rest of the season. Even soap sprays are worth avoiding for the same reason, and the mechanical methods work well enough on a container-sized plant that you rarely need more.
Sticky leaves and black sooty film on the water surface mean an aphid colony above, even if you cannot see it. Look under the rolled tips of the newest leaves.
Mosquitoes, algae and the water itself
A tub of warm still water is ideal mosquito habitat, and larvae will appear within days in the growing season. This is a problem you manage rather than solve, and it is worth doing properly because it is the main reason neighbors object to a water container.
Green water and blanket weed are cosmetic. Algae competes for nutrients but does not damage the plant, and it fades once the lotus leaves cover the surface and shade the water. Lift blanket weed out by hand or twist it onto a stick every week or two. Do not use algaecide, and do not respond to green water by emptying and refilling the tub, which just gives the algae fresh nutrients and chills the mud.
| Problem | What you see | What to do |
|---|---|---|
| Mosquito larvae | Wriggling commas hanging under the surface | Add a Bti dunk or granules monthly, or keep a few small fish |
| Green water | Water like pea soup in early summer | Leave it. It clears once the leaves shade the surface |
| Blanket weed | Long green threads around the stems | Twist it out on a stick weekly. No algaecide |
| Black, sour-smelling mud | Rotten-egg smell when you disturb the soil | Organic matter in the mix. Rebuild with plain mineral soil next spring |
| Warm stagnant film | Oily surface scum on hot days | Top up gently and lift the scum off with newspaper laid on the surface |
Rot, and the mistakes that cause it
Rot in the rhizome is almost never a disease that arrived on its own. It is water getting into the plant's air system, and there are only a few ways that happens. The commonest is a leaf stem cut or snapped below the waterline: the hollow tube floods, the water runs down into the segment it belongs to, and that segment turns gray and soft within a week or two. Always cut damaged stems off above the surface.
The second cause is cold. A rhizome sitting in water near freezing, or in mud that ice reaches into, breaks down over winter and you find slime in spring. In cold areas either harvest everything before hard frost or move the container somewhere the mud cannot freeze through.
The third is planting damage. A snapped growing tip, a bruised segment or a rhizome that was allowed to dry and wrinkle before planting will rot in place rather than grow. Rot spreads along the chain, so if you find a soft segment during harvest, do not keep the piece next to it as your planting stock even if it looks sound.
Leaf damage, animals and reading the signs
Chewed semicircles out of the leaf edges are usually beetles or caterpillars working at night; pick them off after dark and the damage stops. Brown blotches with yellow rims on older leaves are leaf spot, which spreads in crowded still conditions and matters very little late in the season. Remove badly marked leaves above the water and thin nothing else.
Larger animals cause more real harm than insects. Birds will stand on the mud and pull at anything pale, and raccoons or similar will dig a shallow container out entirely overnight, snapping the runner in several places. A grid of wire mesh laid over the tub for the first month, until the leaves are up, prevents almost all of it.
Learn the difference between yellowing that matters and yellowing that does not. A single old outer leaf going yellow while new ones keep coming is normal and needs nothing. Whole-plant paleness in midsummer with green veins is hunger. Sudden collapse of several leaves at once, with a foul smell when you stir the mud, is rot in the rhizome, and the only useful response is to lift it, cut back to clean firm tissue and replant a sound piece with its tip intact.
Note the date you first see aphids, the date any leaf damage starts, and what you did about it. Pests arrive at much the same point in the season each year relative to the weather, and a two-line note this year lets you check the new leaves a week early next year, which is when hand removal still works.