Guttation

What you are looking at

Drops of clear water hanging from the tips of the leaves at six in the morning. On a monstera or an alocasia they can be large enough to fall and mark the floor. On a lawn or a strawberry bed they sit as a rim of beads all round each leaf edge, catching the light.

Two details separate this from dew. The drops are at the margins and the tips rather than spread over the whole surface, and they are on the plant alone while everything around it stays dry.

By the middle of the morning they have gone, and where a drop dried rather than fell there is a small white or faintly brown crusty spot left at the leaf edge. That crust brings more people to this page than the drops do, because it looks convincingly like scale insects.

The plant is pushing, not leaking

During the day a plant moves water by pulling. Evaporation from the leaves puts the whole column under tension and drags water up from the roots, which is what transpiration means.

At night the stomata close and that pull stops. The roots do not stop. They carry on actively loading mineral ions into the xylem, water follows those ions in by osmosis, and with nothing evaporating at the top the pressure has nowhere to go. Root pressure builds until the sap is forced out through the only openings available.

Those openings are hydathodes, small permanently open pores sitting at the ends of the veins at the leaf margin and tip. They are modified stomata that have lost the ability to close, and their job is exactly this. So guttation is a designed release valve rather than a failure of one.

The conditions it needs are warm moist soil, a plant with a working root system, and air humid enough that nothing evaporates overnight. All three together describe a well-watered houseplant in a warm room, or a lawn before dawn in late spring.

The crust, and the brown tip that follows

What comes out is dilute xylem sap rather than pure water. It carries dissolved minerals, a little sugar and some enzymes, and when a drop dries in place the water goes and the solids stay.

Repeat that every morning for a few months on the same square millimetre of leaf and the concentration builds up. The tissue right at the tip eventually dies and browns, which is one of the several routes to a brown leaf tip on a houseplant that is otherwise being looked after well.

Feeding heavily makes it worse, since a stronger feed produces a saltier sap. Cutting the feed rate is usually enough to stop the marking, and wiping the tips off every few days deals with the rest.

When it is telling you something

Guttation on its own is not a fault. Guttation every single morning is worth reading, because it means root pressure is building strongly every night, which means the roots are sitting in more water than the plant is using.

That is the same condition that produces root rot, fungus gnats and yellowing lower leaves, so a plant weeping heavily every morning is worth a check of the compost, the drainage and the saucer before anything else.

Crops have their own reason to care. Several bacteria get into plants by swimming in through the hydathodes, and the guttation fluid gives them both the route and the medium. Black rot in brassicas and bacterial blight in rice both enter this way, drawn back in through the pore when the drop is reabsorbed at sunrise. That is the argument for spacing a brassica bed properly and for watering the soil rather than the foliage, and it is why a still humid night followed by a warm morning is when those diseases spread.

What to do about it

Ignore it where it happens occasionally on a healthy plant.

Water less, and check the drainage, where it happens every morning.

Move some air across a crowded group of houseplants.

Halve the feed if the tips are crusting and browning.

Wipe the deposits off the tips with a damp cloth.

Put a tray or a mat under a large plant that drips onto a wooden floor.

Space brassicas and water at the base rather than over the leaves.

Keep pets away from drips on the floor under an aroid.

What it gets mistaken for

Honeydew from aphids, whitefly, scale or mealybug is sticky, lands over the upper surfaces of the lower leaves and coats whatever sits beneath the plant. Look above the sticky leaf for the insects producing it.

Scale insects are firmly attached lumps that need scraping off and sit along the veins and stems. A dried guttation spot wipes away with a cloth.

Hard water deposits whiten the pot rim and the compost surface too, and mark the leaf wherever a splash landed.

Dew covers everything in the vicinity, including the pot, the table and the window.

Oedema raises corky blisters on the underside of the leaf, which is the same imbalance expressed in a way that does not go away by lunchtime.