Apart from corn and a few sterile ornamentals, grasses are the great enemy of the garden. Few other weeds makes a garden look so messy so fast, or can be so hard to get rid of.
There are two easy times to stop large-seeded annual grasses: in fall when they are about 6″ high, with three leaves or less; and in late spring when they are flowering and forming seed. At these times, they may be easily pulled from soft soil. Between these stages, they are apt to break off rather than pull the roots, or they will come out with a great mass of roots and soil.
Smaller annual grasses like annual rye are more easily pulled as they bloom simply because they are so small in the early stage. But they are apt to start dropping seeds before their roots degrade enough to pull them without soil. Fortunately, their seed is fine and easily smothered with mulch. Large grass seed will come up through considerable amounts of mulch, so they have to be killed at the seedling or flowering stage.
Perennial grasses can easily be killed at the early seedling stage, but that stage is apt to be a lot smaller and harder to catch than in large annual grasses. Once their roots are established, they often have to be dug out. They store food in their roots before flowering and going dormant, so pulling at flowering time only gets the top; the crowns and roots do not easily pull.
The roots start growing again in fall as the seeds come up, and this is a time when many of them can be pulled from soft, newly moistened soil. As winter wears on and the soil compacts as the roots grow, it becomes harder to get them out without digging. Mulching with leaves can prevent compaction and smother some of the smaller plants.
It is said that any plant can eventually be killed by regularly taking it to ground level, preventing it from maintaining its roots. It’s a good theory in most cases, but not for most perennial grasses. They store their food in runner roots or crowns, and their earliest growth can be very hard to spot.
Once you have an established patch of any size, the only real way to get rid of them is digging or poisoning with glyphosate (Roundup), which is likely to kill the plants that they have invaded. In some cases, it may be cheaper and easier to replace the good plants than to dig them up and separate the grass roots from theirs before replanting them. Perennial grass roots are relatively easy to spot and pull from other roots, though, and some grasses, like Bermuda and fine fescue, may require two or more sprayings to kill it all. Digging out the roots gives instant results.