How to know when you’ve watered enough

Lee with his soil-moisture probe (Lee Reich)

Yes, little water has dropped from the sky this spring here in the Hudson Valley (as of this writing, at the end of May, at least – all of which could have changed by the time this goes to press). But a drip irrigation system automatically waters many of my plants. So why have I been spending so much time with hose in hand?

Not all my plants drink in the drips. Trees and shrubs are on their own, except for their first year in the ground, when I religiously hand-water them every few days initially, and then once a week throughout the season. These plants get three-quarters of a gallon per week for every square foot spread (estimated) of their root systems. That’s equivalent to an inch of rainfall – which, if it does fall, exempts me from a few days of watering.

A couple of inches’ depth of hay, leaf or wood chip mulch around the trees and shrubs seals in moisture to make best use of my efforts. Also, I start with smaller plants – less than four feet tall – which become independent of my watering sooner, because a larger proportion of their roots are soon foraging around in surrounding soil than those of larger plants.

My flowerbeds also don’t get dripped. Although the soil surface is dry, moisture carried over from winter still sits in lower depths, into which established perennial flowers’ roots can tap. Annuals and newly planted perennials need to be watered on the same schedule as young trees until their roots reach that moisture.

It is my garden vegetables that drink in the benefits of my drip system. But even here some hand-watering is needed these days. Down each bed run one or two driplines, with emitters along the lines spaced six inches apart. As water enters the soil, capillary pull from small spaces between the soil particles draws water sideways and, along with gravity, downward. The resulting wetting fronts have the shape, if you could look at a cross-section of the soil, of an ice cream cone. In clay soils, with small particles and hence a lot of capillary draw, that ice cream cone is very fat; in sandy soils, it’s narrow: a couple of feet wide at its broadest, as compared with the six-foot spread in a clay soil.

The wetting fronts start their sideways spread below the soil surface – deeper in sandy soils, more shallow in clay soils. In either case, the soil surface remains dry except right at the point of drip. So any vegetable transplants or seeds that I set in the ground need to be hand-watered until their roots reach the wetting front – except for seeds or transplants set right under or along the dripline.

My friend David was wondering why the leaves of his romaine lettuce plants flopped down. I gave my usual response to most gardening questions: “Too little water.” (My other usual response is “Too much water,” often following my first response if the questioner tells of watering all the time.)

So I asked David how much he watered, and he said that he thoroughly soaked the ground by spraying it with water. Busted! It really was a water problem: too little, in this case.

In fact, thoroughly wetting the soil with the usual four-foot-diameter handheld spray is almost impossible. “Thoroughly wet” means soaking the ground to at least a six-inch depth. For his handheld sprayer to do that, David would have to stand in place like a statue, sprayer in hand, unmoving, for about an hour to wet one four-foot-diameter part of the garden before moving on to the next four-foot-diameter area.

When I’m watering plants in the ground by hand, I’m wetting only the small area beneath an individual plant – just enough to soak its roots as they establish themselves in the surrounding soil.

People find it hard to believe that that statuesque watering posture is really necessary. All you have to do is scratch the soil surface after a Davidesque spraying of plants to see how deeply the water percolated, and you’d find only a thin layer of wet soil, at the surface.

Digging a hole in the ground is a good way to tell if watering was sufficient. But it’s also inconvenient.

For just a few dollars, I invested, years ago, in an electronic gizmo that bypasses all that hole-digging. This soil-moisture sensor has a metal probe that you plunge into the soil. Atop the probe is a dial or digital readout that tells whether the soil is “DRY,” “WET” or something in between. More accurate sensors cost over a hundred dollars, but the cheap ones are fairly accurate and work well if coupled with observation.

The soil-moisture meter is especially useful with potted plants, which might need watering every day when the weather is warm, sunny and breezy. (With experience, lifting a pot to feel its weight is also a good measure of moisture level – as is just getting to know your plants better.)

I’m still hand-watering the pots because I haven’t yet connected the drip tubes that will direct water to each of the pots.

 

Any gardening questions? E-mail Lee at [email protected] and he’ll try answering them directly or in his Almanac Weekly column. To read Lee’s previous “Gardener’s Notebook” columns, visit our website at HudsonValleyAlmanacWeekly.com.

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