The Difference Between Annual, Biennial, And Perennial Weeds (& Why It Matters So Much)

You've likely used the terms, "annual," "biennial," and "perennial," while referring to plants in your garden, and unwittingly banished weeds to a completely different category. However, weeds are just as much plants even if they are not always welcome in your yard. And like all plants, weeds, too, can be categorized based on their life cycle and growth patterns. Annual weeds, such as rescue grass and crabgrass, only survive a single year; biennials, such as the bull thistle, which are commonly found in no-till fields and pastures, live for two; and perennial weeds such as quackgrass or the creeping Johnsongrass live for more than two years. The reason this classification is crucial is because correctly identifying their growth patterns often dictates how you get rid of weeds in your garden.

Weeds are a nuisance for a number of reasons. They compete with the plants in your garden for resources, reduce crop yield, and can also spread diseases. There are several types of weeds that attract pests to your yard as well. Nonetheless, even in their notoriety, not all weeds are created equal, and different varieties respond differently to weed management strategies. For instance, shallow cultivation and mowing can help with eliminating upright-growing, annual weeds but fail against deep-rooted, spreading perennials. Such a conundrum makes having a profile of different weed types important, so that you can employ the right methods to get rid of them at the right time.

Hand-pulling can be very effective in controlling annual weeds since their root systems are not as well-developed and hardy as the perennials. But think twice before letting your weeds get too big before pulling in the hopes that you can weed once and get rid of them for good. Annual weeds can produce an enormous quantity of seeds once they grow, and this reseeding ability makes them come back again and again. These seeds can stay dormant for long periods of time and tolerate extreme conditions before germinating again. This makes it essential to get rid of annual weeds while they are still young. Some seeds will sprout again nonetheless, so you must make the practice of hand-pulling a weekly habit to keep the weeds in control. 

You can also use mulch to stop the seeds from germinating. Adding a layer of mulch blocks their access to sunlight and prevents the weeds from ever bothering you in the first place. Pre-emergent herbicides can achieve the same purpose. Furthermore, since annual seeds are most susceptible during their seedling stage, contact herbicides—which are a kind of post-emergent herbicides that kill the part of the weeds they get in touch with—can also be effective against annual weeds during this stage.

Like annual weeds, biennial weeds produce a lot of seeds as well. These seeds are also easily transported by the wind, thanks to the presence of a fuzzy substance on them called pappus—a feature that makes the weeds difficult to eradicate. To control biennials, you must spray them with herbicides as late in the fall season as possible to hit the maximum number of seedlings that have emerged by then. You can tackle the ones that emerge afterwards later in the spring as they will remain vegetative until then.  

Perennial weeds are the most difficult weeds to control because of their hardy root systems as well as their ability to reproduce vegetatively and spread underground. These features allow them to grow year after year, and make pre-emergent herbicides ineffective on them. In fact, these weeds persist even if you remove the part of the vegetation that is visible aboveground.

Furthermore, their incredible rate of growth during spring and summer makes them resistant to the temporary action of most systemic herbicides as well (ones that are absorbed by the roots of the plant). This is why one of the most effective ways to control perennial weeds is to use systemic herbicides against them during the fall season. During fall, the natural tendency of plants to start sending resources from leaves to roots amplifies the action of these herbicides, which also work in the same direction, and makes controlling them easier. Nonetheless, it may still take up to three weeks for these herbicides to work and eliminate the weeds.