A2: The weeds that have appeared in your new lawn are commonly occurring, opportunistic species. Such weeds often come up on newly disturbed soil, where they can rapidly colonise and establish themselves. They will have originated from seed which has lain dormant in the soil, possibly for many years, or from weed seed that may have been wind-blown, or introduced by birds. Some of these weed species will disappear by regular mowing. Others might need to be treated once the new lawn has been well established. All grass seed is examined for purity before it is released for sale.