Watering vegetables right: the finger test, morning watering, pots vs beds

Watering right does not mean watering on a timetable. It means watering when the soil starts to dry, watering deep enough to reach the roots, and letting the soil breathe before the next round. The simplest way to know when is to push a finger into the soil, and the best time is the morning, at the base of the plant.

This follows the soil pH lesson, which ended on the point that yellow leaves often come from water, not from the soil.

The finger test, every time

• Pots: push your index finger in to the second knuckle, about 5 cm. If your fingertip feels cool and moist, do not water yet. If it comes out dry with little soil stuck to it, it is time.
• Beds: dig 5-10 cm down with a trowel. Moist soil holds together in a loose clump; dry soil falls apart like dust, or in clay is so hard it is difficult to dig.
• Lift the pot: for small pots, weight helps. Lift it right after watering to learn the feel, then compare on later days. A pot that has become much lighter is dry.

A dry surface does not mean the soil below is dry. The top always dries first, especially in strong sun.

Water in the morning, at the base

A University of Kentucky plant pathologist recommends watering in the morning because the foliage dries quickly, and, where possible, watering the soil near the base rather than the leaves. Leaves that stay wet for long, especially overnight, are what many fungal diseases like.

In the Thai hot season, small pots on a sunny balcony may be dry by afternoon. A top-up in the early evening is fine, but water the soil and leave time before dark.

Beds and pots need different watering

	In-ground bed	Pot
How fast it dries	Slowly; water can move into deeper soil	Fast; little soil, wind and sun on all sides
Typical frequency	Deep, less often	Often every day in the hot season
How much	About 25 L per m² per week, adjusted for rain and sun	Pour slowly until water drains from the holes
Main risk	Shallow watering keeps roots near the surface	Water pooling at the bottom, root rot

Clemson University says vegetables in a summer garden need about 1 inch (about 2.5 cm) of water a week, rain included. That amount wets the soil 15-20 cm deep, where most vegetable roots are. The figure comes from the US; a hotter Thai hot season may need more, so always check with your finger as well.

For pots, Oregon State University notes that they dry out fast and daily watering is common, especially for pots on concrete in full sun. Water until it runs out of the drainage holes, but do not let the soil stay soggy or let water pool on top. Texas A&M’s guide adds that leafy, heavy-foliage container plants may need watering twice a day at times, and less during slow growth.

Worked example: a small 0.5 x 1 m bed

1. Area = 0.5 × 1 = 0.5 square metres
2. 2.5 cm of water over 1 square metre = 100 × 100 × 2.5 = 25,000 cubic centimetres = 25 litres
3. A 0.5 m² bed needs about 12.5 litres a week if there is no rain
4. Split into 3 waterings a week, about 4 litres each, or most of a 5-litre watering can
5. Before each watering, dig 5-10 cm to check; if it is still moist, skip to the next day

Pour a little, let it soak in, then pour more. Very dry soil sheds the first water, which runs off the side of the bed instead of sinking in.

A thin mulch of straw or dry leaves, and compost dug in before planting, help the soil hold moisture for longer.

Signs of too little and too much water

Sign	Too little	Too much
Soil	Dry and crumbly, pot feels light	Always soggy, never dries out
Leaves	Wilted; young leaves dull, darkened or greyish	Yellow, then dropping
After watering	Perks up within a few hours	Does not recover, or gets worse
Roots	Dry	Dark brown to black, soft, mushy

Clemson warns that if you wait until leaves wilt before watering, the plant may already be damaged and the yield reduced. Too much water starves the roots of air, turns leaves yellow so they drop, and opens the door to root rot.

Wet soil and a wilting plant: do not add water. Suspect the roots first.

If you suspect overwatering, stop until the top layer of soil dries, check the drainage holes and empty the saucer. If a potted plant keeps getting worse, ease it out and look at the roots. Healthy roots are pale and firm. If most of them are black and mushy, the plant usually will not recover.

A simple self-watering pot

A wicking container has a water reservoir under the soil, and the soil draws water up by capillary action, the way a cloth with one end in water slowly gets wet all the way up. A University of Kentucky guide explains the principles:

• An overflow hole sets the water level. Drill it in the side at the top of the reservoir. Any extra water runs out, so the soil above never sits in water.
• Water rises about 10 cm above the level of the overflow hole, so the bottom of the soil must be in contact with the reservoir.
• A fill tube. A PVC pipe reaches down to the reservoir; you fill through the pipe instead of watering the surface.
• Cap the tube. An open fill tube attracts mosquitoes, so make a removable cap.

A simple version you can build: two buckets, one inside the other. The outer one holds the water. Drill several holes in the base of the inner one and stand it on spacers so its base is lifted off the outer one’s base. Let some soil reach down into the water through the holes, or through strips of cloth as wicks. Drill the overflow hole in the outer bucket just below the inner bucket’s base, and add a fill tube.

These pots help a lot when you are away for a few days, but you still need to check the water level, and in the rainy season make sure the overflow hole is not blocked.

Common mistakes

• Watering by the clock, not the soil. The same every morning in both the hot and the rainy season.
• Watering only the surface. A light sprinkle every day wets the top while the soil below stays dry, so roots stay shallow and wilt easily.
• Blasting the leaves in the evening. The leaves stay wet overnight, and a hard jet washes soil away until roots show.
• Leaving water in the saucer. The roots sit in water, and mosquitoes breed there.
• Watering the moment a plant wilts. Check the soil first. If it is still wet, the problem is the roots, not the water.

Sources

• Watering the Vegetable Garden — Clemson Cooperative Extension HGIC
• Proper management early on can lower disease risks in gardens — University of Kentucky
• No yard? Grow vegetables in containers — Oregon State University Extension
• Overwatering — UC Statewide IPM Program
• Self-Watering Wicking Container (AEN-157) — University of Kentucky Extension
• Vegetable Gardening in Containers (E-545) — Texas A&M AgriLife Extension