Horticulture/The Importance of Studying Weeds

While they're not often a favorite topic of study for the horticulture student or enthusiast, knowing and understanding weeds is one of the most important keys to becoming a successful gardener. Aside from being a maintenance issue (in the sense of always needing control), weeds also play a key role in the ecosystem which the gardener seeks to manage.

What is a weed? This can be hard to define, but a good general definition is that a weed is any plant growing in a place where it is not wanted. Using this definition, the same plant can be both a weed and a crop at the same time, to the same gardener, on two different areas of the ground. For example, clover may be planted on a pasture and considered a forage crop, but at the same time it may appear in the vegetable garden and considered a weed. This also means that the gardener can change his mind about what constitutes a weed over time: that self-seeding basil that escapes to invade the neighboring dill becomes a weed as soon as it crosses the border.

Life cycle
Like any plants, each particular weed has a distinct life cycle. First and foremost, it's important to know whether or not the weed is perennial, since perennial weeds require complete removal or destruction, while annuals and biennials can be controlled using preventative techniques such as pre-emergents or mulches.

Propagation, persistence, and tolerance
Knowing how a weed spreads is also important. Many perennials spread by stolon or rhizome, while other plants (including all annuals and biennials) spread primarily by seed.

How persistent a weed is depends on its ability to grow back from small fragments and the life span of its seed. Certain perennial weeds can grow back from even the smallest root fragment, and the seeds of some weeds can survive dormant in the soil for decades.

A weeds tolerance of abuse such as regular mowing can also provide clues to the easiest possible control techniques.

What to know when deciding whether to control

 * Are they serving as a placeholder on what would otherwise be bare soil (and thus perhaps could be colonized by an even worse weed)?
 * Are they likely to colonize other areas if left unchecked?
 * Are they serving as nectar sources for pollinators and beneficial insects?
 * Are they serving as alternate hosts for pests and diseases?
 * Are they serving as trap crops, attracting pests that would normally eat your crops?
 * Are they providing home for predatory insects?
 * Are they going to seed soon?

Garden uses
Some plants which are usually treated as weeds are intentionally grown in some gardens, so it is important for professional gardeners to be aware of this so that they can ask the client whether they should be removed. Conversely, it's important to know that a plant can be weedy when considering using it in the garden.

Native range
When working in "native plant gardens", it is important to know whether a "weed" is native to the region, since weeds can in those situations be treated as "volunteers".

Pests and diseases
While it's tempting not to worry about pests and diseases that are affecting garden weeds, many weeds serve as alternate hosts for pests and diseases that affect garden plants. In order to provide lasting control for pests and diseases, it may be necessary to either eradicate these alternate hosts, or in the case of pest controls it might be necessary to control the pest on the weed hosts as well as the garden plant hosts, to prevent them from just migrating back to the garden plants after treatment.

On the other side of the spectrum is the use of "trap crops", popular in organic gardening, to draw pests away from garden plants. If a type of weed is getting eaten up by insects, it is likely serving this role. Note that this is only effective if the garden contains sufficient predator populations. Some weeds can serve as both traps and hosts. Lamb's Quarters, a common garden weed across the United States, is a trap for leaf miners but a host to beet leafhoppers.

Nectar value
Weeds in uncultivated areas can be useful in attracting beneficial insects.

Toxicity and edibility
Some weeds contain toxins, ranging from those that cause discomfort to those which are lethal even in small doses, so it is important to be aware of these in gardens where children and pets play.

Many weeds are also edible, and while they tend not to be choice ingredients (if they were, they'd be cultivated), they can be of interest. Some weeds, such as Purslane, are renowned for their nutritional value.

Tools
Many tools are available for weed control. There are hand or powered hoes, flame weeders, hand or powered cultivators, scythes and similar hand cutting tools, mowers, all the way up to tractor-mounted flail mowers for knocking down weedy brush at the edges of meadows or pastures.

Soil types
Some weeds can serve as indicators for the soil type where they are growing, and can be "read" to provide a preliminary description of the soil type when a soil test has not yet been performed.

Disposal
whether it is safe for cold compost piles, hot compost piles, etc.