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Washington State University Extension

Garden Tips

THINGS GARDENERS SHOULD KNOW

written by Marianne C. Ophardt WSU Extension Faculty for the Tri-City Herald, Kennewick, WA LANDSCAPE FABRIC: Twenty years ago, many area landscapes were mulched with black plastic and rock mulches. This combination made it particularly tough to grow trees and shrubs successfully. The black plastic didn’t allow for the free movement of water and air into the soil. Plus, the plastic combined with the rock mulches increased soil temperatures and greatly stressed plants. The black plastic didn’t last indefinitely and weeds became a problem again before long. Eventually ‘geotextiles’ came along and started to replace the troublesome and restrictive black plastic. These woven or spun materials allowed for the free movement of air and water. They also were effective in reducing germinating weeds. Today, geotextile landscape fabrics have become the standard in landscaping, but they have not been the ultimate answer to weed management. Landscape fabrics do have some drawbacks. First, they do degrade over time, especially when exposed to sunlight. Ones that are UV resistant will last longer. Usually a top layer of mulch is applied over the fabric, increasing its life and the appearance of a landscape bed. If an organic mulch (my preference) is used on top of the fabric, eventually the mulch starts to decompose and will become colonized by weeds. Weeds are kept out longer if inorganic rock mulches are used… but eventually weeds will show up whether organic or rock mulches are used over the fabric. Removal of weeds in these situations is made difficult because the weeds’ roots grow into the fabric. Also, the roots of landscape plants tend to grow near the soil surface and grow into the fabrics. When the fabric breaks down and removal is desired, plant roots are damaged. While the use of geotextile fabrics can deter weeds for a some time, it’s better not to use them. They don’t provide permanent weed control in permanent landscape situations. Organic mulches, such as shredded bark, coarse compost, or wood chips, are the best choice for managing weeds in home landscape beds. They are attractive, their decomposition nurtures beneficial soil organisms, and their breakdown helps improve the soil. PRUNING PAINT: ‘Wound dressings’ or ‘wound paints.’ have been around for years and have been used by conscientious, caring gardeners trying to keep their plants healthy. It was thought that these wound dressings would stop wood rot and prevent the entrance of decay organisms and boring insects when painted on tree wounds. Unfortunately, the various wound dressing preparations do just the opposite. Wound dressings seal in moisture, actually creating a better environment for the decay organisms. Some of the materials used delay callus formation and the closing of the pruning wounds, increasing the amount of decay that can occur. They can also increase the amount tissue damage at the wound site, increasing the overall injury. Plus, these materials do eventually dry out and crack, making it easy for pathogens to find their way in. The use of wound dressing has not been recommended by tree care professionals and horticulturists for many years. Products containing collagen, pectin, hydrogel, and aloe gel have all been touted in recent years as natural compounds that will enhance tree healing. Unfortunately, there is no evidence that these new materials or the old materials hasten healing or prevent decay. The best way to promote healing of pruning wounds is to make proper pruning cuts and then leave the wound open to light an air. For other physical wounds to trunks, it’s important to make a smooth edge to the wound and leave it open to air and light. There is no good reason to apply a wound dressing or wound paint to pruning cut, trunk injuries, or cracks in the trunk. PHOSPHORUS FERTILIZERS AND FLOWERING: No doubt you have seen specialized fertilizers on the garden store shelf. Some of these are ‘formulated’ to promote flowering of annuals and flowering shrubs, such as roses. Most of the flower enhancing fertilizers contain higher levels of phosphorus. You can probably find a good number of references and web sites that recommend the application of phosphorus fertilizers to promote flowering, but the truth is that extra applications of phosphorus will do no good if there is an adequate level of phosphorus already present in the soil. Most of our local garden soils have adequate levels of phosphorus, especially if someone has applied fertilizers containing phosphorus in the past. Phosphorus does not readily leach out of the soil, as does nitrogen. By applying excess phosphorus you will decrease soil health. By decreasing soil health, you decrease plant health. Weaker plants are more subject to attack from diseases and insects and susceptible to nutrient deficiencies… setting up a greater dependency on chemicals. So don’t add extra phosphorus from flower promoting fertilizers unless you know your soil is deficient in this nutrient. Is your soil deficient in phosphorus? Get a soil test done to find out. LAWN MOWING: Mowing… it’s the most tedious lawn task, but it’s also one of the most important. Mowing at the right height, the right frequency, and the right way go a long way to providing you with a nice looking, healthy lawn that’s able to resist invasion by weeds and attack by pests. Kentucky bluegrass lawns mowed too high will develop thatch more quickly and will lose a nicely manicured look. Mowing too low is even worse than mowing too high. Low mowing decreases the leaf surface area, decreasing the photosynthesis needed for plant growth. This leads to weaker grass plants with shallower, weaker root systems. This makes the grass plants more prone to stress from possible drought and heat stress during the summer. The recommended mowing height for Kentucky bluegrass lawns in our region is 2 to 2.5 inches. Lawns should be mowed often enough so that no more than 1/3 of the leaf blade is removed at any one mowing. ‘Scalping’ the lawn by removing more than 1/3 of the leaf blade will injure and weaken grass plants. If you go away on vacation and the lawn gets away from you, mow first at the highest height you can achieve on the mower. Once those clippings are dry, mow again at the recommended height in a different direction. (However, it would be better to have someone mow in your absence.) For the best looking lawn, mow your lawn in a different direction every one or two mowings. Mow at right angles to the previous mowing. If you always mow in the same direction, the grass will have a tendency to be pushed in that direction and it won’t stand up well. Be sure to keep your mower blade very sharp. Dull blades tear the ends of the grass, which weakens the grass plants by damaging more leaf tissue and leaves a lawn with an off-color, less manicured appearance.

Published: 6/4/2005 1:40 PM

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