How to Water Herbs: When, How Much and How to Tell

Most advice about watering comes as a timetable, and most timetables fail, because no two gardens dry out at the same pace. Learning how to water herbs is really learning one small habit: check the soil first, then decide.

This guide gives you that check, explains how it differs between thirsty herbs and drought-tolerant ones, and shows the watering method that gets moisture down to the roots where it counts.

Why a calendar does not work

How quickly an herb uses up the water around its roots depends on a long list of things, and they change from week to week:

• Soil type. Sandy soil drains and dries fast. Clay holds water much longer.
• Rain. A steady soaking rain can supply everything a bed needs for a good while. A brief shower may wet only the surface.
• Heat, wind and sun. Hot, breezy, bright days pull moisture from leaves and soil far faster than cool, still, cloudy ones.
• Mulch. Covered soil loses less water to evaporation than bare soil.
• Plant size. A large, leafy plant drinks more than a small one.
• New or established. A new transplant has shallow roots. An established plant reaches deeper reserves.
• Ground or container. A pot holds a limited amount of mix and dries much faster than a garden bed.

As an example of how wide the range is, a small pot of basil in a heat wave might need water daily, while an established thyme in the ground may go weeks without any. A fixed schedule would be wrong for at least one of them.

The finger test

The reliable way to decide is to feel the soil. Push a finger 1 to 2 in (2.5 to 5 cm) into the soil beside the plant, past the surface layer, which can look dry while the soil below is still moist.

• If it feels cool and damp at that depth, wait and check again later.
• If it feels dry at that depth, it is time to water herbs that like steady moisture.
• For drought-tolerant herbs, dry at that depth is only the starting point. Let the soil dry further before you water.

In very hot or windy weather, check more often. After rain, check before assuming the job has been done for you. Under mulch, move the mulch aside and test the soil itself.

Know which herbs like moisture and which like it dry

Herbs split into two groups, and treating them alike is where much trouble starts.

The moisture lovers have soft leaves and grow fast, so they flag quickly when the soil dries. The second group prefers leaner, drier conditions, and constantly wet soil is a far bigger risk to them than a dry spell.

Drainage decides how forgiving either group is. Herbs in well-drained soil cope with a generous watering, while the same amount in heavy, compacted ground can leave roots sitting in water. The guide to the best soil for herbs explains how to improve drainage before it becomes a watering problem.

How to water: slow, deep and at the base

When the finger test says it is time, water properly instead of a little.

• Water at the base. Aim the stream at the soil around the stem, not over the leaves. Foliage that stays wet for long periods is more prone to leaf diseases.
• Go slowly. A gentle flow soaks in. A fast one runs off across the surface and takes soil with it.
• Go deep. Keep going until the moisture has reached well down around the roots. Deep watering encourages roots to grow downward, where the soil stays moist longer.
• Prefer the morning. Plants start the day with a full supply, and any splashed leaves dry quickly as the day warms.

Avoid frequent light sprinkles. They wet only the top of the soil, so roots stay near the surface where they dry out fastest, and the plant becomes more dependent on you, not less.

New transplants need closer watching

A newly planted herb lives on the small root ball it arrived with, whatever its type. Until new roots grow out into the surrounding soil, even rosemary or thyme can dry out quickly.

For the first few weeks, check the soil right at the root ball often, and water when it begins to dry. As fresh growth appears and the plant becomes established, lengthen the gap between waterings and go back to the normal finger test for that herb.

Wilting does not always mean thirst

Soft-leaved herbs such as basil often droop in strong afternoon heat even when the soil holds plenty of moisture. The leaves are simply losing water faster than the roots can supply it, and the plant commonly recovers by evening.

So check the soil before you water a wilting plant. If the soil is dry, water. If it is moist, wait and look again in the cool of the evening or the next morning.

If a plant keeps wilting or yellowing whatever you do, the guide to common herb garden problems walks through the other possibilities.

Mulch helps the soil hold water

A layer of mulch shades the soil, slows evaporation and softens the impact of heavy rain. Around basil, parsley and other moisture lovers, a couple of inches (about 5 cm) of an organic mulch such as shredded leaves or straw means the soil stays evenly moist for longer.

Keep mulch a little back from the stems. Around rosemary, thyme, sage and oregano, use it sparingly or choose gravel, so the base of the plant stays dry.

Herbs in pots dry out faster

Everything above applies to containers, only faster. A pot is exposed to sun and wind on all sides, and its roots cannot reach beyond the potting mix. Small pots and porous clay pots dry quickest of all.

Check potted herbs more often than those in the ground, especially in summer, and water until it runs from the drainage holes. Never let a pot stand in a saucer of water for long. The guide to growing herbs in containers covers pot size, potting mix and drainage in detail.