LSU AgCenter's Weekly Message

Hot, Dry Times (we may have had rain in the last few days; however, this information remains helpful for some and for all of us in the future)

As of August 24, the US Drought Monitor showed all areas of Tangipahoa and Washington Parishes in severe drought. You may recall that we had a period of moderate drought last fall, as well, though it wasn’t as hot then.

A rule of thumb is that plants need roughly 1 inch of water per week. In reality, water needs vary according to temperature, soil texture, plant type and size, and other factors. Under current conditions, I think a better estimation for many planted areas would be around 1.5 to 1.75 inches of water per week. However, it’s important not to overwater plants, too.

If you use a sprinkler, you can place several tuna or similar cans (make sure they have straight sides) in the area the sprinkler covers to find out how long you need to run it to provide 1 inch of water. Run the sprinkler for a set amount of time and then average the depths of the water in the cans to find out how much water was put out in that time. If you don’t open the spigot completely, note what position the handle is in when you do this, so that you can open it the same amount the next time.

When watering vegetables, lawns, and other smaller plants with a sprinkler, it’s best to do it in the early morning so that water can soak into the soil without much evaporation and plants can dry as the sun rises. Watering in the evening or at night helps conserve water, too, but if water gets on leaves in the evening it may stay on them all night. Long periods of leaf wetness increase the chances of plants being infected with various fungal and bacterial pathogens.

If you’re not watering with a sprinkler, it may be less obvious how to deliver 1 inch of water. Since we usually think of water in gallons, here are the amounts of water needed to cover several example areas to a 1-inch depth: 1-foot diameter circle, 0.5 gallon; 3-foot diameter circle, 4.4 gallons; 10 square feet, 6.2 gallons; 100 square feet, 62 gallons; 1000 square feet, 620 gallons, and 1 acre, 27,154 gallons.

If you use a hose to water, you can time how long it takes to fill a 5-gallon bucket to figure out how long it takes to provide a given amount of water. Be sure to turn the handle on the faucet to the same place each time to get approximately the same flow rate you measured.

The frequency with which irrigation is needed – i.e., into how many applications the needed volume of water should be divided – depends on how deeply rooted the plant is and how well-drained the soil is.

Lighter, more sandy soils need less water at one time and more frequent applications than heavier soils do. Sandy soil cannot hold as much water. If too much is applied at one time, some will drain below the root zone of the plants. On the other hand, if you put too much water at one time on a heavier soil, water may accumulate on the surface and run off the area in which it’s needed, so watch out for this.

If you’re watering a vegetable garden, it’s probably best to apply some water every day or every other day, instead of just once per week. Since most vegetables are annuals, they don’t have time to develop very deep root systems. For tomatoes and other vegetables that sometimes get blossom end rot, it’s important to keep the amount of water in the soil consistent, since plants don’t take up calcium without adequate water being present.

On the other hand, a large, established tree should have roots that allow it to access water more deeply. If soil isn’t very sandy, watering once per week should be sufficient.

Of course, whether you’re watering with a sprinkler or just with a hose, chances are you won’t be able to water the tree’s entire root system at one time. LSU AgCenter urban forestry specialist Dr. Hallie Dozier has recommended placing the end of a hose somewhere within the area between the edge of the tree’s canopy and about 2 feet from the trunk, letting it drip slowly for 6 to 8 hours, and then moving it to another spot within the root zone. Water should not be allowed to run off the area to which it’s applied. If runoff occurs, the flow of the hose needs to be reduced, or the hose needs to be moved.

If you have a soaker hose that will extend around the root zone of a tree, this can be allowed to drip for approximately 24 to 72 hours, unless runoff occurs.

It might seem that watering every day or every other day, with small amounts of water each time, would be ideal, but in most landscape situations, this is not the case. Watering “deeply and infrequently” rather than “shallowly and frequently” is important for encouraging deep root growth for young plants and for getting to the roots of established trees.

Keep in mind that mulch helps conserve soil moisture. A depth of 3 to 4 inches is appropriate for most situations, but don’t allow it to rest directly against the bases of plants.

Let me know if you have questions.

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Dr. Mary Helen Ferguson is an Extension Agent with the LSU AgCenter, with horticulture responsibilities in Washington and Tangipahoa Parishes. Contact Mary Helen at mhferguson@agcenter.lsu.edu or 985-277-1850 (Hammond) or 985-839-7855 (Franklinton).