Few weeds detract from an attractive carpet of green grass quite like tall, prickly thistles. These common, aggressive weeds can quickly take over entire lawns and gardens. Various thistle cultivars reproduce from seeds and underground roots, making them difficult to eradicate. Controlling these weeds in the landscape requires vigilance and prompt action.
Examine your lawn to identify specific areas of thistle growth. Thistles, like other weeds usually take over bare areas of soil where grass grows poorly, such as the area near curbs or against the outside perimeter of your yard that may not receive sufficient amounts of water to keep the grass healthy.
Water any large areas of thistle that have taken over the soil in specific areas. Water deeply to soak the soil. Wait 24 hours to allow the water to absorb completely. Dig up the thistles in these areas that contain only minimal amounts of grass. Pull out the long taproots, using caution to avoid breaking the roots. Rake the soil smooth and scatter new grass seed over the top to replace the thistles and discourage the growth of new weeds. Keep the soil moist as the new grass seed sprouts and grows.
Apply a selective foliar herbicide to the entire lawn early in the spring when you notice the presence of thistle rosettes in your lawn, but prior to the appearance of flowering stalks. Select a type of herbicide safe for killing thistles in lawn grasses. These products include ingredients such as metsulfuron, triclopyr or dicamba, broadleaved herbicidal chemicals that won't harm lawn grasses. Mix and apply the herbicide according to the label instructions and apply it to the lawn on a dry, calm day. Repeat the application later in the season if you notice the formation of new thistle rosettes appearing in your lawn.
Improve the health of your lawn grass to help it withstand weed and pest infestations. Apply a nitrogen fertilise to your lawn grass once or twice each season. Using a product that also contains a broadleaved herbicide will help knock down thistles and other weeds at the same time. Keep your lawn cut to a length between 2 and 3 inches long to discourage the flower formation and seed disbursement in existing thistles.