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If at all possible, get weeds out before they cast their seeds. Identifying the plant can help. In fact, being able to identify weeds is vital, because how you manage them depends on what sort of weed your garden has. An inexpensive, and very helpful book is Ronald Taylor's Northwest Weeds, Missoula Montana, 1990. This book shows photos of all the most vigorous and pesky annual and perennial weeds.
How can you know which weeds are annual? That means the weed finishes its life span in one season, sprouting, growing, and going to seed without wintering over. Annual weeds survive by making seeds in the thousands.
If you want to protect water and water quality, think of mechanical means of control for these weeds. The best methods for dealing with annual weeds all involve persistence---hoeing, mowing, shading out with crops or ground covers, covering the ground with geotextiles or mulch or both. Be vigilant. Some of the nastiest annual weeds such as shepherd's purse (Capsella bursa-pastoris) spread vigorously from seed during the late summer.
Some gardeners believe that weed-killers (herbicides) are the best answer for all types of weeds, but this isn't true for annual weeds. We are all concerned about protecting water quality, and herbicides should be chosen carefully. The most commonly used one is probably glyphosate ( sold as Round-Up and other brand names). But this weed-killer works by entering the system of the plant and killing it from the root up. It's useful on some perennial weeds but not a good idea on an annual weed that is going to die out at the end of the season.
Indeed, some annual weeds treated this way with systemics will die but will produce their seeds while dying, thus defeating the purpose of getting rid of the weed. Hoeing the weeds out and covering the ground with plants or mulch works better.
What about spreading an herbicide on the ground to keep seeds from germinating? Many products are sold for this purpose. It's important to consider the use of the area before selecting one of the herbicides called "pre-emergent," named for the fact that they keep seeds from coming up. These products work, but they have to be used very carefully. Some are best avoided because they essentially sterilize the ground for a year or more. (Check the labels on products with common names such as monobor chlorate and prometon--these are soil sterilants. Washington State University weed specialists don't recommend these because they move into water tables so readily and cause pollution. Avoid these. Many a gardener has found out the sorrow of treating a piece of land for weeds and then discovering nothing at all will grow there.
Probably the most common product sold for pre-emergent control is dichlobenil, with the brand names Casoron or Norosac, and others. Use this product carefully if at all. It's only labeled for use around plants that have woody root systems and have been established in the ground for at least one growing season (6 months or more.) It can't be used where the gardener intends to plant, or has planted, annuals, perennial flowers, vegetables, bulbs, or new woody shrubs or trees. If over-applied, dichlobenil can make it impossible to use the ground because plants will not emerge.
If you plant hardy bulbs such as daffodils (planted in fall), and then spread Casoron over the surface of the soil, the bulb shoots will be killed or damaged and you won't get any flower production.
Choose mulches and other weed barriers and use herbicides exactly according to the label, and on the appropriate weeds.
Hortsense: Managing plant problems with Integrated Pest Management
