Underwatering (Drought Stress)
Underwatering is letting the rootball dry out so the tree can't supply water to its foliage.
Underwatering (Drought Stress) at a glance
| Also known as | Drying out, drought stress |
|---|---|
| Affected parts | Leaves/needles, fine roots |
| Most at risk | Moisture-lovers and small/shallow pots in heat (maples, hornbeam, larch, tea tree, katsura, bahama berry) |
| Main cause | Soil allowed to dry out completely; missed watering in heat/wind |
| Severity | Fast, fine roots can die in hours for sensitive species |
| Reversible? | Often, if leaves haven't fully crisped and fine roots survive |
What to look for
- Leaves: Wilting/drooping, then dry, crispy, brown leaf margins or whole leaves; needles browning from the tips. Sudden leaf drop after a hot day.
- Soil: Dry, pale, and lightweight; may have shrunk away from the sides of the pot so water runs straight down the gap.
- Roots: Fine feeder roots desiccate and die; the rootball may be hard and dry throughout.
- Whole tree: Limp overall, often worst on the side facing sun/wind.
Overview
Underwatering is letting the rootball dry out so the tree can't supply water to its foliage. Some species are forgiving, but many bonsai, and especially those with fine roots in small, shallow pots during hot or windy weather, can be badly damaged, or killed, by a single complete dry-out. A few species (e.g. tea tree, bahama berry, katsura) are notoriously unforgiving of even brief drought.
Causes
Skipped or too-light watering; hot, dry, windy weather drying pots fast; a hydrophobic/dried-out rootball that repels water; root-bound trees with little soil; very free-draining mix not watered often enough.
Commonly confused with
Overwatering also causes wilting/yellowing but with wet soil. Leaf scorch from sun/wind looks similar but the rest of the soil/tree is well-watered. Fertilizer burn browns margins too, but follows heavy feeding. The deciding test: dry, light soil and a dry rootball mean underwatering.
Which species are affected
All species, but most dangerous for moisture-loving and fine-rooted trees (maples, hornbeam, beech, larch, willow, katsura, tea tree, bahama berry) and any tree in a small/shallow pot in summer.
Treatment
Water immediately and thoroughly; if the rootball is bone-dry and repelling water, soak the whole pot in a tub of water until bubbles stop, then drain. Move to shade and out of wind for a few days. Don't fertilize a stressed tree. Many trees re-leaf if the roots survived; wait before assuming branches are dead (scratch test for green cambium).
Fix it
What the treatment above uses, from our shop:
- Bonsai Moisture Meter $10.95
- Green Watering Can, 1 Liter $16.95
Step by step: How to Water a Bonsai (Without Killing It)
Prevention
Check soil daily in the growing season (twice on hot days); water until it runs from the holes; use soil with enough moisture retention for the species; shelter sensitive trees from midday sun and wind; don't let trees become root-bound.