Underwatering

Underwatering is the easiest problem on this site to fix and the easiest to keep repeating. The plant runs out of water in the medium, loses turgor pressure in the leaves, and collapses. Water it and it comes back.

What you are seeing#

Leaves hang straight down from the stem like wet laundry. They feel thin and papery between your fingers, not thick or rubbery, and a badly dried plant has leaves that feel almost crisp at the edges.

The droop shows up late in the light period and is worst right before lights off, because that is when the plant has been transpiring longest. Seedlings and small clones wilt hardest because they have almost no root mass to buffer them.

Lift the pot. A dry 3 gallon fabric pot feels startlingly light. Push a finger in and the medium is dry and dusty, and in soil it may have pulled away from the sides leaving a visible gap.

After a few days you also get yellowing lower leaves and stalled growth, because a dry root zone cannot move nutrients even when they are present.

Why it happens#

Most often the grower is scared of overwatering, which they have read is the classic beginner mistake, and overcorrects into never giving a full soak.

The other causes are mechanical. Watering a small volume that only wets the top inch. Soil dried so completely it repels water, which then runs down the gap at the edge of the pot and out the bottom while the center stays dry. A root bound plant in a pot it outgrew three weeks ago. Or a hot, dry tent and a big plant drinking far more per day than it did last week.

How to fix it#

1. Water now, slowly. Pour in a circle, pause a minute, pour again. You want 10 to 20 percent of what you put in to come out the bottom.
2. If the soil is hydrophobic, meaning water beads up or runs straight through, bottom water instead. Set the pot in a tray with 2 inches of water for 20 to 30 minutes until the surface darkens.
3. Check back in 2 hours. A thirsty plant is usually standing again by then. If it is not, the problem is not thirst.
4. Set a weight baseline. Lift the pot right after watering and again when dry, then water when it hits the light end. Most 3 gallon pots in veg land on every 2 to 4 days.
5. Over the next week, look at pot size. If you are watering daily and it still wilts, the plant has outgrown the container. Move up to a 5 gallon before flowering.
6. Check the environment. Above 85F or below 40 percent humidity, a plant drinks much faster than you expect and yesterday's schedule stops working.

How to tell it apart#

Versus overwatering. Both droop, and this is the single most common misread in growing. Underwatered leaves are limp and papery and the pot is light. Overwatered leaves are firm and thick and curl downward, and the pot is heavy. Lift the pot first, every time, before you decide which one you have.

Versus heat stress. Heat stress curls leaf edges up into a canoe shape and the plant may still be firm, with a canopy temperature above 85F. Underwatering hangs the whole leaf down and is fixed by water alone. Put a thermometer at canopy height before blaming water.

Versus low humidity stress. Low humidity gives you crisp tips and upward curling edges on a plant whose pot is still wet. Underwatering gives whole-leaf collapse on a pot that is dry.

How long until it looks better#

Fast. Two to six hours for a mild wilt, and almost every plant is back to normal posture by the next lights on.

Leaves that went fully crisp at the edges will not come back and those tips stay brown. Remove any leaf that dried out completely once the plant is stable. Judge recovery on new growth over the next 5 to 7 days, which should come in normal sized and flat. If growth is still stunted a week later, the plant sat dry long enough to kill fine root hairs and needs another week or two to rebuild them.