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Pretty flowers or weeds?

Posted 4/29/2008

Many plants grow really well in Southern Nevada. Any plant that’s growing where we don’t want it in our landscapes or gardens is considered a weed. Many of the weeds coming up in yards and on open lots come up from seeds that fall off dried stems of plants that break loose from the soil and blow around. The most famous example of these is the "tumbling tumble weed". Other plants that are invasive can grow from seeds or spreading roots of existing plants. Really invasive plants like Bermuda grass and Field bindweed (wild morning glory) reproduce by seed and spreading roots.

Weeds have always been a problem in agriculture and horticulture. They increase costs or are a nuisance in landscapes and crop production. In contrast, invasive exotics like Saltcedar are causing significant environmental harm. These species are highly adapted to widespread environments. They out compete native plants, significantly decreasing the native plant and animal diversity. This change results in loss of forage, wildlife habitat and recreational uses, which decreases resource value. A local example of habitat destruction is where Saltcedar has killed out the native willows along the rivers, which created a problem for the now endangered Willow Flycatcher.

The transportation of contaminated hay, seeds, flower arrangements and other apparently harmless purchases has introduced invasive weeds into Nevada. Many of the plants on the State Noxious Weed list were brought into the country and planted because of their pretty flowers, like Dalmatian toadflax and Tall whitetop. Weed parts and seeds hitchhike on vehicles, sports equipment and animals, including humans, from infested areas to un-infested sites. Most people are not aware that they could create a new plant infestation or disrupt native vegetation by bringing in even one plant or seed. When a plant is introduced from a foreign habitat, its natural pests that keep it from spreading normally do not come with it.

Each state has a list of the worst invasive weeds that are legally declared as noxious weeds and property owners can be required to control those specific weeds. Information on Nevada’s noxious weeds is on the Nevada Department of Agriculture web site at http://agri.state.nv.us/ under the Plant Industry section. Control and management is easiest and least expensive if done when weeds are first identified in an area. If they become established and reproduce, their control becomes difficult and very expensive. Cultural, biological and chemical controls are available for many invasive plants. For more information on identification and control of many weeds found in Nevada, go to the University of Nevada Cooperative Extension Web site at www.unce.unr.edu and click on publications and then search for "weed". You can help keep Nevada’s worst weeds under control.

Now back to your own back yard, how can you get rid of the weeds growing in your rock landscape, garden and lawn? You may hear all kinds of tips like use salt or soap and water on them. Most odd tips usually won’t work and can cause damage. Although there are chemicals that will control most weeds, the best alternative weed control methods are the use of mulches, using a hoe, and improving competition from other plants. If you must use weed control chemicals, it would be best to direct these chemicals on the weed itself rather than spraying or spreading chemicals over an entire area. A nonselective weed killer that kills most everything, such as glyphosate (Roundup®) can work well if applied during cooler parts of the year, particularly in the fall. Another chemical, fluazifop (Fusilade), gives good control of grasses in ground covers, shrubs and trees. Broadleaf weed killers like 2,4-D will kill broadleaf weeds in grass without killing the grass. When working with chemicals, always read and follow the label recommendations.

You need to decide whether the weed is an annual (comes back from seed every year) or perennial (comes back from the roots or stems each year). Most annual weeds have a life cycle that causes them to die each year after they bloom and set seed. Examples of this type of weed are the mustards; they germinate in the fall, bloom in the spring and die in the early summer. Other annual weeds are annuals just because they die when it freezes. If we don’t get freezing temperatures, then these types of annual weeds may last through the winter and continue to grow in the spring. Two examples of these weeds are crabgrass and spurge. Annual weeds typically have smaller, shallower roots that are not woody when compared to roots of perennial weeds that enlarge each year and can become woody with age. Control of annuals will be different than perennials. Annual weeds are not good competitors with other plants. Their best defenses against control are producing lots of seed, quick germination of seed and rapid growth. They usually mature rapidly and produce lots of seeds in their attempt at establishing themselves and taking over. They try to get up and established before other plants can. Many annual weeds will produce seed in 30 days or less after they germinate. That’s why it is so important to control annual weeds as soon as you see them. Mulching and using a hoe are excellent ways of controlling annual weeds. A hoe that cuts the stems of annual weeds just below the surface is very effective and does not disturb the roots of other plants growing in the area. The secret for controlling annual weeds is weekly hoeing in the areas where they are a problem. If you know where you will have annuals coming up, you can use a pre-emergent. You must apply it before the seeds germinate. Pre-emergents are nonselective, so only use them in areas where you are not planning to grow other plants from seed such as in established turfgrass or open space areas.

Mulches are very effective on annual weeds. Using mulches in planter beds, vegetable gardens, under fruit trees and in desert landscapes is a good idea. Mulches act much like a competing plant to help reduce the germination of annual weeds. They also can help to keep the soil cool and conserve water. Mulches can be wood chips in planter beds, compost in vegetable gardens, milled rock or gravel in desert landscapes, plastic (in some cases like vegetable gardens, but never under rock), shredded paper, untreated grass clippings and the like.

The non-application of water can be a very effective weed control tool in the desert. Applying water only where it is needed, using drip irrigation and scheduling it properly, can be very effective at controlling weeds that might otherwise grow between emitters if sprayers were used. In turfgrass, raising the mowing height, fertilizing regularly and keeping it watered and healthy can control annual weeds very easily. Most weeds don’t grow well in grass that is thick and dense. Avoid edging at an angle along sidewalks and around sprinkler heads. Spurge and Bermuda grass move in anywhere the seeds can reach soil and get sunlight and water, then spread into other areas.

Perennial weeds usually represent a very small percentage of our weed problems, but once they get established they are the most difficult to control. Examples are common Bermuda grass and Nut grass. These weeds usually have to be controlled with weed killers. But you can reduce the amount of weed killer you use just by taking the time to direct the chemical on the plant rather than applying it over the entire area that is infested. A spray bottle containing 2,4-D directed at dandelions as you see them growing in the lawn is just as effective as applying this chemical to the entire lawn as a weed and feed.

Weeds will be your most serious maintenance problem with desert landscapes using rock. Most of the time the rock layer is thick enough to prevent many annual weeds from coming up through it. Cooperative Extension does not recommend using black plastic under the rock. It will be a problem for you as it disintegrates and starts poking up through the rock and becomes unsightly. If you need to put some sort of barrier down under the rock, landscape fabric is a better choice. The fabric is put down first, before the rock, and is meant to keep weeds from growing through the rock from the soil. Barriers will not keep weeds such as Bermuda grass from invading, and dirt/sand will blow into the landscape. This dirt/sand will be a soil layer for the establishment of weeds.

One of the best methods of controlling weeds in the landscape is to apply water only where the landscape plants are located. Our intense, dry summers do a great job in controlling many weeds in areas where there is no irrigation. Of course where drip emitters feed water to plants also is the best spot for weeds to grow. These will always be weedy spots. I would try pulling out the shallow rooted annual weeds first. Weeds such as common Bermuda grass will have to be sprayed with chemicals to get it under control. The University of Nevada Cooperative Extension is an outreach arm of the University that extends unbiased, research-based knowledge from the University of Nevada—and other land-grant universities—to local communities. Educational programs are developed based on local needs, often in partnership with other agencies and volunteers. For more information about the University of Nevada Cooperative Extension, please visit the website at www.unce.unr.edu or call (702) 397-2604 or 346-7215.

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