Weeds are plants out of the proper place. A plant may be desirable in one location and an undesired (weed) in another. For example, in lawns any plant other than our desired turf grass is weed. Weeds detract from the beauty of lawns due to the dissimilarity in color and texture between the desired grass plants and the weeds. In addition, weeds compete with the desired grass plants for space, available water and nutrients, usually resulting in thinning, poor growth and ugly look of desirable plant cover.
Lawn weeds may classify in two classes based on the way in which they germinate from seed or emerge from the soil. Monocots emerge with a single seed leaf whereas dicots emerge with two seed leaves. Most monocot weeds found in lawns are from the family Gramineae and are termed grassy weeds or narrow leaved weeds. Examples of grassy weeds in Pakistan include crabgrass, barnyard grass, hooraghass, green foxtail, annual bluegrass, tall fescue and quackgrass. Dicots, on the other hand, are termed broadleaf weeds and include such plants as horse pursalane, spurge, garden spurge, dandelion, clover, ground ivy, knotweed and plantain.
Effective control of weeds in turf is based on correct identification. Many books and charts are available to help in identifying common lawn weeds.
The most effective method of controlling lawn weeds is to maintain a dense and vigorously growing turf cover. Weeds are often an indication of problems in the grass plant environment, and killing the weeds without correcting the underlying problem will lead to unsatisfactory results. Often turf weeds can be controlled simply by altering the cultural practices to favour the grass plants rather than the weeds. The cultural controls may include raising or lowering the mowing height, changing the frequency of mowing, lengthening or shortening the period between irrigations, increasing or decreasing the application of fertilizer, or aerifying the soil. A combination of proper cultural practices plus prudent use of chemical is sometimes necessary to control weeds effectively in turf. Chemicals which are used to control weeds are called as herbicides or weedicides. There are two major types of herbicides, pre-emergence and post emergence herbicides. Pre-emergence herbicides affect germinating seeds. To be effective, the herbicide should be applied two to three weeks before weed seeds germinate. Consequently, pre-emergence herbicides are most effective against annual weeds. For control of summer annuals such as crabgrass, application of pre-emergence herbicides between March 10 to April 20 in Punjab, Pakistan. Post-emergence herbicides are used to kill weeds after the weed plants are up and growing. To be effective, most post-emergence herbicides must be absorbed through the leaves; consequently, liquid sprays generally work better than dry, granular materials. However, granular formulations may be the most practical way for homeowners to apply these materials.Post-emergence herbicides are most effectively applied when weeds are young and growing vigorously. Selective post-emergence herbicides are usually used to control annual, biennial, and perennial broad-leaved weeds because they will kill many broadleaf plants without damaging grass plants. For example 2, 4-D; these herbicides can severely damage or kill trees, shrubs, and flowers; thus, they should be used with great care near these plants.
Nonselective post-emergence herbicides kill all plants, both desirable and undesirable. These herbicides can be used to spot treat perennial grassy weeds that are not affected by selective herbicides. To spot treat an area, thoroughly wet the weed foliage with herbicide solution.
If you have severe problem of weeds in your lawn simply use round up (glyphosate) it is nonselective herbicide. It will kill all weeds including your desired lawn grass within 3 weeks. Than cultivate, watered and cover this area with plastic sheet (during sunny and hot days) for 8 to 10 days to kill weed seeds present in soil seed bank. After this reseed or transplant your desired lawn grass. Here we are giving few advices to prevent further built up of weed problem. When your lawn is stressed, its ripe for takeover by weeds. Therefore, deep watering and infrequently helps your lawn competes by encouraging deeper root growth. Set your lawn mower higher: Mowing at the proper height, usually one of the 2 highest settings on your mower helps the grass grow thick to shade weed seeds, so its harder for them to grow. Proper Feeding: Feeding at regularly, like every 6-8 weeks during the growing season helps your lawn thicken up and fill in bare spots, making them less welcoming to weeds.

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