What makes a weed a weed?

The answer is a little complicated. At the most basic level, a weed could be any plant growing where it’s not wanted, according to the Bureau of Land Management. By this definition, even desirable plants can be weeds when they spread beyond their intended habitat.

If the black-eyed Susans you plant in a landscaped bed escape their container and spread into your yard, technically they’ve become a weed.

But weeds aren’t all bad. In some ways, they are beneficial. For example, they offer nectar for bees and serve as food and habitat for wildlife, according to the Penn State Extension.

Take dandelions. They are one of the first plants to bloom each spring, which makes them an important food source for insects when not much else is in bloom. Weeds also benefit the soil, too, helping to stabilize it and adding organic matter.

And not everything we call a weed is a weed. Take milkweed and Joe Pye weed. Despite their common names, they aren’t weeds at all. Native milkweed and Joe Pye weed are beneficial plants; their names are just misnomers.

Prevention

As they say, an ounce of prevention is worth a pound of cure. The most pernicious weeds are widely adaptable — they don’t seem to mind poor soil, periods of drought or soggy soil and competition from other plants. So how do you control these backyard bullies?

Keep disturbance to a minimum. Soil that’s been dug up, raked, rototilled or otherwise disturbed brings up weed seeds that may be lingering in the ground, waiting for some light to germinate.

Eliminate free and clear conditions. Bare ground is an open invitation to weeds. The space you leave between your flowers and shrubs is an opening for weeds until the garden plants mature and cover it over or shade it. It’s best to lay down several inches of mulch between the plants.

Completely cover open areas with several layers of newspaper or cardboard. Then add 2 to 4 inches of mulch, wood chips or soil on top. This method helps smother developing weeds, depriving them of sunlight and the water needed to survive.

Battle tactics

Some effective weed-controlling methods:

Dig or pull weeds out, early and often. Get every last bit because some weeds will regrow from root fragments.

Cut off flower heads before they go to seed.

Mow down the area repeatedly, starting early in the season, when the weeds start to poke up and before they go to seed. Repeat again and again if they regrow. Eventually this assault will deprive the roots of food because you chopped off the leaves and stems that energize them and they will die.

Smother the area. When starting a new plot, use plastic sheeting for six to eight weeks during the hottest part of the year. It will smother the weeds.

Use an herbicide specifically labeled for control of your weed, and always read the label carefully before using. The label will give you the application instructions, including the best time of day and time of year, the waiting time before you can replant the treated area, and dosage amounts. Weed killers may be applied in various ways, including spraying on leaves, painting onto leaves and stems, or spraying over an entire patch. Protect nearby garden plants with a tarp; work on a dry, windless day; and wear protective gear.

Proper disposal

Do not underestimate the enemy. Never toss weeds or weed fragments onto your compost pile, or even into the woods or brush surrounding your property.

Some compost piles get quite hot, but this may not be sufficient to kill weeds or weed seeds. It is best to bag all weed parts and put them in the trash, or you could burn them.

What about lawn weeds?

Growing healthy, dense turf is the best way to prevent weeds. Vigorous, healthy turf shades weed seeds so they don’t germinate. Select suitable lawn grasses, fertilize properly (primarily in fall), lime as needed based on soil test recommendations, mow frequently (removing only one-third of the leaf tissue with each cutting), and water during establishment and only to keep lawn green during drought.

When you water, be sure to thoroughly soak soil weekly. Frequent, light watering encourages weed encroachment, discourages deep rooting and lowers the environmental stress tolerance of turfgrasses.

Aerate and add compost to improve soil structure. Mowing at a height of 3 inches or more will help shade out weed seeds and prevent them from germinating.

Chemical control for lawns

Herbicides, which kill or alter the normal growth of weeds, can be divided into two main groups: selective and nonselective. Selective herbicides control the target weed without damaging desirable turfgrasses. Nonselective herbicides kill all vegetation (including turfgrasses) and are used in lawn renovation or for weeds not controlled by selective herbicides.

Herbicides can be further divided into preemergence and postemergence categories. Preemergent herbicides are applied prior to germination and weed emergence. They are typically used for controlling annual weeds. Postemergence herbicides are used for controlling weeds that are already growing.

They are systemic or contact in nature. Contact herbicides affect the plant parts they touch and are not translocated to other parts of the weed. Systemic herbicides are translocated throughout the plant, so they are effective at killing perennials: weeds that can generate new foliage from underground roots.

Herbicides can be applied to foliage or soil.

Postemergence herbicides are usually applied to leaves, and preemergence herbicides are applied to soil. A foliar-applied herbicide must be absorbed by the leaves. It is less effective if washed off the leaf surface by rainfall or irrigation.

Soil herbicides can be applied as liquids or granules and should be watered into the ground (by rainfall or 15-20 minutes of irrigation) following application. When using any chemical treatment, read the label. It is a contract.

Dee Dee Kerscher is a Penn State Extension Master Gardener volunteer and horticulture program assistant.