In fall, the root systems of deciduous trees as well as warm- and cool-season grasses go into a sort of hyperdrive – rapidly increasing their uptake of water and nutrients before the cold weather arrives.
Deep irrigation during late October and early November will help ensure the vigor of trees and lawn grasses next spring. A fall fertilization of fruit and nut trees (not including citrus) will replace nitrogen that was lost when crops were harvested. Late fall is also the best time to apply systemic insecticides to trees that have had severe infestations of aphids and to treat invasive Bermuda grass with glyphosate herbicides such as Roundup.
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Instructions for deep irrigation of mature trees were given in this column a few weeks ago. Here’s a quick review.
▪ Deep irrigation should slowly soak the tree’s entire root system to a depth of about 12 inches; most of a tree’s root system extends about a foot deep into the soil and to the edge of the canopy. Place soaker hoses, bubblers, small oscillating sprinklers or multiple drip emitters at the edge of the canopy where the younger feed roots lie – not at the trunk – and allow the water to run for 3 to 4 hours once a week.
▪ Fruit and nut trees need one feeding in fall with a higher nitrogen fertilizer. Ammonium sulfate (21-0-0) is cheap and effective; high-nitrogen lawn foods are more expensive but they work well. When applying any high nitrogen fertilizer, be careful to avoid runoff onto hard surfaces or into gutters that might carry the nitrogen into our water supply. Irrigate the soil under the trees before applying the fertilizer, scratch the fertilizer into the soil surface and irrigate again.
▪ Crape myrtles, Chinese elms and hackberry trees are often susceptible to aphid infestations. The aphids excrete sticky “honeydew” that can be a nuisance, and a severe infestation can weaken the tree. The decision to use systemic insecticides to treat for aphid infestations is not an easy one. Most systemic insecticides have a neonicinotoid or neonic as the active ingredient and neonics are considered to be a major factor in the decline in bee populations. Aphid populations can be reduced with applications of horticultural oils in winter and that should be the first treatment tried. A neonic systemic insecticide applied in fall as a soil drench will be drawn up by the root system and reach the leaf canopy in early spring. Although some insecticide labels suggest yearly applications, research has shown that the insecticide can remain effective for three to four years.
▪ Bermuda grass spreads by three means: seeds, rhizomes and stolons. That’s why it’s so hard to kill completely. Many gardeners who replaced their Bermuda lawns with drought-tolerant landscaping these last few years still find small persistent tufts of Bermuda grass popping up. The Bermuda rhizomes are actively storing water now. Paint the Bermuda grass blades with a glyphosate herbicide. The herbicide will translocate or move to the active rhizomes and have a strong effect. Because of concerns that glyphosate herbicides are carcinogenic, wear gloves and don’t spray – especially on windy days. You can expect to make several applications for the herbicide to be effective.
Send Elinor Teague plant questions at firstname.lastname@example.org.