Bottom Watering vs Top Watering: Which, and When

Top watering means pouring water onto the surface of the medium until it runs from the drainage hole. Water travels downward through the pot, wetting what it passes and carrying dissolved material with it, and the surplus leaves through the base.

Bottom watering means standing the pot in a few centimetres of water and letting capillary action draw it upward through the medium. Water moves from the bottom of the pot toward the top, and nothing leaves.

That difference in direction is the whole comparison. Top watering flushes; bottom watering does not. Top watering wets the surface first and the base last; bottom watering does the reverse. Everything else follows from those two facts.

Both methods, done properly, thoroughly wet the root ball. Neither is a shortcut, and neither changes how often the plant needs water — that is set by the plant, the light and the medium, not by how the water arrives.

It solves the hydrophobic compost problem completely. Peat-based compost that has dried out repels water, so top watering runs down the gap at the pot wall and out of the drainage hole while the root ball stays dry. Standing the pot in water bypasses this entirely — capillary action pulls water into the dry medium rather than over it.

It wets the whole root ball evenly. Top watering, done in a hurry, often wets the top third and leaves the bottom dry. Bottom watering cannot do this, because the water starts at the bottom.

It keeps the surface dry, which matters more than it sounds. Fungus gnats lay their eggs in the top two or three centimetres of damp medium, and a surface that stays dry between waterings interrupts their life cycle. For anyone with a recurring gnat problem, this alone is a reason to switch.

It keeps water off the foliage and out of the crown, which is essential for African violets and other fuzzy-leaved gesneriads that mark permanently where cold water sits, and for rosette plants where water in the centre causes crown rot.

And it is harder to overwater in one sense: the plant takes what it can hold and no more, provided you remove it from the tray when it stops absorbing.

Water carries dissolved salts — from fertiliser, from hard tap water — and those salts have to go somewhere. When you top water thoroughly, they leave through the drainage hole with the surplus. When you bottom water, they cannot leave, so they travel upward with the water and are deposited at the top of the pot as it evaporates.

Over months this produces a genuine accumulation in the upper layer of the medium, which is often visible as a white or yellowish crust on the surface and around the rim. The consequences are the classic salt symptoms: brown crisp leaf tips and margins, reduced growth, and roots damaged by the raised salt concentration in the zone where many of the fine feeder roots sit.

This is not a reason to avoid bottom watering. It is a reason not to bottom water exclusively, and the fix takes a minute.

Every fourth or fifth watering, top water thoroughly instead — slowly, with two or three pot-volumes of water, letting it drain right through and discarding the run-off. That single habit carries the accumulated salts out and resets the pot.

It is also worth top watering when the surface crust is visible, whenever you have been feeding regularly, and always if you water with hard tap water.

For bottom watering: stand the pot in a tray, sink or bowl with 2–5 cm of tepid water — enough to reach the bottom third of the pot, not to submerge it. Leave it 15–30 minutes. Check the surface of the medium; when the top has darkened with moisture, it is done. Lift it out, let it drain for a few minutes, and return it. If the water has not been absorbed after 30 minutes, the plant did not need it.

For top watering: pour slowly and evenly across the whole surface, not in one spot, and continue until water runs freely from the drainage hole. Wait a minute and repeat — the first pass often runs down the pot wall, and the second is what actually soaks the medium. Then empty the saucer or cachepot completely.

Two details apply to both. Use tepid water rather than cold, particularly for tropical plants and especially for African violets. And judge the interval by the medium, not by the method — checking with a finger or a skewer is what decides when to water, regardless of how you then do it.

One caution specific to bottom watering: if a plant has fungus gnats or any soil-borne problem, do not share a tray between pots. Standing several pots in the same water spreads whatever is in one of them to all of them.

Bottom water as the routine for small and medium pots, especially if you have fungus gnats, dry-out-prone compost, or plants that mark when their leaves get wet. It is more reliable at wetting the root ball evenly, and it removes a class of common errors.

Top water thoroughly every fourth or fifth time to flush accumulated salts, and top water as the default if you live in a hard-water area or feed heavily.

For large pots, top watering is the only practical option, and that is fine — the flushing it provides is one of its advantages.

The most important point is the one the debate obscures: how you water matters much less than whether you water thoroughly and at the right interval. A plant soaked properly once a week by either method will do far better than one given a splash every three days by whichever method is currently fashionable.