Watering Wisely Defeats Weeds & Disease!
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The basic rules of watering are very simple: Never water ANYTHING in the evening or at the height of the day. Early morning is always best.
'Why' has a lot to do with basic plant physiology. Dew forms as evening progresses to morning, and plants have developed the ability to let their cells 'open up' to accept some of this water through their leaves. (This is the basic principle behind foliar feeding, when you spray something like compost tea directly on plants to feed them, which you should only do in the early morning). The plants' roots are also most receptive to water in the morning, and will utilize any moisture you provide with the most efficiency.
Morning is the only time its relatively safe to wet the leaves of disease-prone plants like tomatoes, roses and lilacs, because the rising sun will quickly dry those leaves off before disease spores can use that moisture to breed. That said, I STRONGLY suggest you only wet those leaves for a good reason, like foliar feeding. It's always better to water plants at the base; with the disease-prone, it's often the difference between life and late blight.
As the day progresses, the physics change. The plant cells that actively tried to take in as much water as they could in the morning start to close up to retain that moisture. By high noon on a blazing hot summer day, those cells are closed up tighter than Dick Cheney's wallet at a gun control fundraiser. Watering then makes absolutely no sense. At the very least, you're wasting the maximum amount of water—the plants' roots are absorbing as little as possible, and you're losing the maximum amount to evaporation. And if it's a 100-degree day and you soak the poor plants with 45-degree water from a deep well, they might get a little bit stressed, don't you think?
Yes, those cells do open up again when the sun goes down, but this is the worst time to wet the leaves of your plants, especially those disease-prone ones; and lawns. Think about it—if you soak your garden down when you get home from work (when its probably most convenient foryou) those plants will stay damp for a good 12 or 14 hours before the sun arrives to dry them off. By then, disease will have had its way with them.
Here's Watering 101: Get a rain gauge and chart what Nature provides. An inch of rain a week is perfect for every plant in your landscape in most climates most times of the year. Yes, if you live in the deep South, have desperately sandy soil and/or we're getting hammered with an Al Gore heat wave, you can certainly move up to two inches. But one inch or two, be sure and apply it correctly; and that's in big lumps, not short spurts.
Frequent shallow sprinklings insure puny root growth, which is bad for all plants. It's also the second biggest cause of persistent weed problems in lawns (the first is scalping). Frequent watering also risks the plants' roots staying wet, which can rot them right there in the ground. Plants need to dry out between watering. Here's the ideal scenario:
It pours rain from 10 pm Sunday night to 8 AM Monday morning. Then the sun comes out and dries off all the leaves. Monday the soil is visibly soaked. Tuesday the soil is visibly soaked. By Wednesday, the very top inch of the soil will seem dry; but sit on your hose—those roots are still in saturated ground. By Thursday, the tops of those roots are finally beginning to dry out a little. Feels good. On Friday, they're 100% in dry soil, so they grow downward in search of water, strengthening the entire plant. Saturday, they grow a little deeper, maybe find some water, but basically stay dry, protecting them from root rot. Then on Sunday night it pours again. Now that's a happy landscape.
If possible, you do the same. After a week without rain, set your sprinkler, soaker hoses or irrigation system to come on around midnight and end around 8 AM. Your rain gauge will tell you if you need to adjust the time to achieve an inch ending right as the sun comes up. (It takes a LONG time to put an inch of water down). It's OK to wet those leaves at night if you don't stop at night; the moving water will protect the plant and then the rising sun will dry the leaves. (Although ground level watering is always better.)
Now, this does not apply to new plantings of big items, like trees and shrubs. When you put these things in the ground in early Spring or Fall (NEVER in the Summer; they will die, guaranteed), let a hose drip water at the base of each new planting for a full 24 hours, and repeat every few days for several weeks if rain is scarce. Back off if rain is plentiful, but be ready to supply a several hour 'drip drink' a couple times a week for a full year when it isn't. Lack of water early on is the biggest cause of death for newly planted trees.
And of course, have an inch or two of compost, shredded leaves or other non-wood mulch around (but never touching) your plants to keep moisture in the soil.
And finally, if we hit a summer where it never stops raining, remove the mulch and clap if you believe in Fairies; maybe Tinkerbell will show up and dry your plants off with them little wings.