Overhead Watering

You’ve probably heard me say it a hundred times – overhead watering is bad.

But how can overhead watering be bad when plants have been rained on for a very long time? Let’s find out.

Watering with a garden hose (PxHere)

In most cases when it rains, the sky is overcast and temperatures are cooler. [Or you’re watering tropical plants that have evolved their own set of built-in protections.] When most gardens get watered with a hose, it’s a sunny afternoon, after you get home from work. Herein lies the problem.

There you are, standing in your garden with a hose in your hand after a long day at the office. You’re sprinkling vegies and other plants with life-giving water. Dust washes away and your plants seem to be refreshed from above and below. But you may be causing more harm than good with overhead watering.

It’s all a matter of timing because when leaves stay wet at night they are far more likely to develop fungal and bacterial diseases.

Furrow irrigation (LBM1948) CC BY-SA 4.0

Disease contributor

Overhead watering is a major contributor to fungal and bacterial diseases, such as anthracnose, bacterial spot, black rot, black spot, downy mildews, early blight, halo blight, and leaf spot, just to name a few. Plus, if you have one sick plant, droplets from overhead watering can bounce pathogens onto healthy plants, spreading disease.

These diseases all need three conditions to occur at the same time for your plants to get sick: a susceptible plant, a pathogen, and the right conditions. This is called the disease triangle. Remove any one of the three and your plants stay healthy.

​A wasteful practice

Overhead watering is wasteful. On a hot day, as much as 30% of the water ends up running off or evaporating, providing your plants with nothing but disease potential. Some of that evaporation occurs before the water ever reaches the plant. The rest of it happens when the water lands on sun-warmed leaves. Add a little more sunshine and that irrigation water has left the building. Overhead watering also contributes to erosion and groundwater contamination.

Drip-emitter watering a container plant (Alan.ca) Public Domain

What’s in your tap water?

The chemicals found in your tap water also have an impact on plant health. Here, in San Jose, Ca, my soil is alkaline clay, with a pH of 7.7 and our tap water has a pH range of 7.0 to 8.7. I don’t care how often I work on acidifying my soil, irrigating with alkaline water is not going to help. If your tap water is part of your gardening problem, you may want to supplement your irrigation with rainwater collected in rain barrels.

Irrigating options

To make the most of your irrigation water, you first need to know your soil texture. Heavy clay can hold on to a surprising amount of water and should be watered less often. Sandy soils need frequent watering. Next, time your irrigation in such a way that leaves can dry out before nightfall.

Soaker hose in a raised bed (PxHere)

Besides sprinklers and overhead watering with a garden hose, consider these options:

• drip systems – very efficient, but they tend to get clogged in areas with hard water.
• furrow irrigation – lay a hose at one end and let the furrow fill up
• hand watering at ground level – very efficient, allowing each plant to be watered according to its needs; time-consuming (though very pleasant)
• soaker hoses – provide a slow release of water, excellent for raised beds, tree rings, and traditional garden rows
• sub-irrigation systems – highly efficient underground watering systems; expensive

Conservative watering also reduces weed growth and helps keep nutrients in the soil where your plants can reach them. Mulch helps, too.

There are times when overhead watering can be helpful. Black spot fungal spores take 24 hours to fully attach themselves to leaves. Some growers have found that if spores are washed off of plants every day during peak infection periods, the spores can’t infect plants. A quick afternoon rinse can also get rid of dust, which provides habitat for aphids and spider mites. It can also help cool off plants that are heat or wind stressed. Again, the important thing is making sure the leaves have time to dry out before nightfall.

When you water, water deeply. This encourages roots to grow downward where they are more likely to find their own water and be safely away from various types of surface exposure. And be sure to select plants suited to your microclimate, minimizing the need for irrigation in the first place.

People used to say that watering in the afternoon would cause leaves to burn. That somehow the sun’s rays would use water droplets as magnifying glasses and burn leaves. This isn’t true. Burnt leaves are usually the result of too much fertilizer, chemical overspray, or misapplication of dormant oils.

If you really love watering your garden with a hose the way I do, just do it in the morning. This will give leaves time to dry off during the day.

​How do you water your garden?