How much water do plants really need? Would you know if you were overwatering your plants?

The amount of time needed to sufficiently water your plants depends on how much and how often you water, the root-zone depth, the weather and the type of soil.

A dry soil surface is not always a sign of water need and may not be a true indicator of what is going on near the plant root. Additionally, most plants will normally wilt in the hot sun and may recover on their own later in the day as the sun moves.

To tell if a plant needs water, feel and smell the soil. Within the first 2 to 3 inches the soil should feel mildly moist. Soil that has been too long without oxygen usually smells sour or rotten.

Plant root depth has a major impact on how often plants need water. Efficient and effective watering will supply water to the full depth of the plant’s roots (but no deeper) every time you water.

New plantings and seedlings will require more frequent watering than established plants since they have not had sufficient time to establish deep root systems.

Shallow-rooted plants like lettuce, spinach, mustard and radishes require less water than deeper-rooted plants, such as tomatoes, but may need to be watered more frequently.

For deep-rooted plants, water less frequently but for longer periods, so that water reaches deep into soil. Frequent, shallow watering for deeper-rooted plants encourages undesirable fine surface roots that can be easily injured by cultivation or drought, or burned by fertilizers.

Water should percolate 8 to 12 inches into the soil for most garden plants. Some of the larger, deeper-rooted plants, such as tomatoes, may require wetting 12 to 18 inches into the soil.

Environment also plays a role in plant water use. Hotter, windier and full sun areas require more watering than cool, protected, shady areas.

In soils where water drains quickly, plants will need to be watered more frequently. Incorporating organic matter, such as compost, into these soils will increase their water-holding capacity.

Overwatering plants can lead to the damage or death of a plant. Plant roots require oxygen to breathe and when soils are waterlogged, oxygen is limited and roots are unable to grow and function well.

Some signs of overwatering are:

>> Stunted or slow growth of a plant

>> Yellowing of young leaves

>> Wilting of young shoots and stem

>> Leaves that are green but brittle

>> Water-soaked spots or blisters on stems or leaves

>> Brownish-colored or rotting roots

Most water-related problems with plants can be eliminated by making simple changes to your watering practices. Learning to water efficiently and effectively is not only good for your plants but will also save water.

Cynthia Nazario-Leary is an urban horticulture extension agent and master gardener coordinator at the University of Hawaii at Manoa. Email her at cnazario@hawaii.edu.