The definition of a weed is: any undesirable, or troublesome plant, especially one that grows profusely where it is not wanted.
Going off the definition a weed can be pretty much any plant. Weeds do fall into two main categories: annuals and perennials. (There are also a few biennials, but they’re generally treated like perennials.) Annual weeds are ones that sprout anew each year, live their entire life cycle in one year and then produce seed for the next generation. Perennial weeds are those that come back year after year. They also may go to seed or send out runners, but unlike annuals, they don’t die out with the season. When it comes to killing weeds you have to be able to determine if it is a grassy weed, or a broadleaf weed, in order to determine the proper control measure.
Common annual lawn weeds: Crabgrass, Goose Grass, Foxtail, Black Medic, Prostrate Knotweed, Prostrate Spurge, Purslane, common Chickweed, Corn Speedwell, Dog Fennel, and Henbit.
Some common perennial lawn weeds: Dandelion, Quack Grass, Tall Fescue, Yellow Nutsedge, Wild Garlic, Plantain, Creeping Speedwell, Cinquefoil, Creeping Charlie (ground ivy), Clover, and Wild Violets
Grassy weeds are generally thin, upright, and recognized by long, narrow, leaf veins that run up and down parallel to one another. Common grassy weeds in turf grass are Crabgrass, Goose Grass, Yellow Nutsedge, and Quack Grass. It’s tough to kill grassy weeds in lawns. This is because these plants are botanically so similar to turf grass that products that kill them will also damage the desirable turf grass. There are a few products that kill grassy weeds after they’re growing, but all work best when applied early in these plants’ growth stage. Unfortunately it can be hard to distinguish grassy-weed outbreaks until they are so far along that grassy herbicides aren’t very effective. Even under ideal conditions, it may take two or three applications to control a grassy-weed problem. This is why it is much more effective to go after annual grassy weeds like Crabgrass and Goose Grass with a preemergent before they’ve sprouted. The good news is annual grassy weeds like Crabgrass and Goose Grass die off when frost arrives. But for perennial grassy weeds like Quack Grass and Tall Fescue, your only options are digging out the patches, or spraying the infested area with a non-selective herbicide such as Glyphosate (round up) and then reseeding the area.
Broad-leaf weeds generally have wider leaves, that have a main vein down the middle, with smaller veins branching out in a netlike pattern. Common broadleaf weeds in turf grass are Dandelions, Creeping Charlie (ground ivy), Wild Violets, Clover, and Thistle to name a few. While broadleaf weeds are easier to control then grassy weeds, some broadleaf weeds need several herbicide applications to start to gain control (Creeping Charlie, Violets) while others, like Dandelions, will show good improvement after just one application.
Herbicides used to control weeds either kill by contact, or systemically. Its recommended to use a combination of both to achieve best results. Contact herbicides start to kill the weed when they come in contact with the leaf tissue. Systemic herbicides trans-locate throughout the plant and kill it from the root up. Always remember weeds are opportunists. They’ll quickly move into any bare spots. The best method to prevent weeds is to follow proper cultural practices and to keep the turf healthy and thick.