There is only a single 100% weed killer for control of long-lived buried seed:- sterilization of the soil with live steam for several hours. This is feasible only in greenhouses, where special covers are used on benches to retain heat and pressure at a lethal level.
Practical control can be had in home gardens or future lawn areas by the use of one of two chemical materials.
Calcium Cyanamide: Used at a rate of 75 pounds to 1,000 square feet of surface (a heavy dose) this has killed out all weed plants and seeds for me, even on fairly heavy clay soil. Calcium cyanamide is a granular material that at first breaks down into substances poisonous to seeds but later converts into valuable nitrogen and lime. It is a grim coffin-gray in color and even looks poisonous to handle, but is safe if used as directed. The soil to be treated should be plowed or rotary tilled and leveled just before application. After 60 days you can plant seed, but disturb the soil surface as little as possible, to avoid bringing up new weed seeds. The 60-day wait is one drawback to this material. Since soil should be warm during treatment, this means you have an unplanted lawn or garden during the major part of the growing period, which some gardeners find too unpleasant a sight to face.
A lawn I treated in this way four years ago came up without a single weed and, except for a few seeds blown in from the outside, has had no weeds since.
Vapam: This is a fumigant, a liquid used in much the same way as garden cyanamid; it works almost as well but for some reason has failed to control purslane, a persistent and annoying weed in my soil. Otherwise, it cleaned out such nasty perennials as Canada thistle and bindweed, as well as all annual weeds. Vapam is somewhat easier to use than calcium cyanamide; soil fumigated with vapam can be reseeded within a week after treatment.
A second fumigant, methyl bromide, is not generally practical for amateur use. However, some landscape gardeners are equipped with a device that injects the gas into soil. The area is then covered with a plastic sheet. A day later the cover is removed to air out the fumigated soil. After a day or two, it is ready for seeding.
Chemical control of annual weeds in flower and vegetable plantings is not always easy. Both Sesone or Crag Herbicide 1, and Alanap are quite effective on selected crops. One difficulty is that they will not discriminate between seedlings of weeds and those of desirable plants.
In a commercial truck garden where a single crop occupies several acres, weed control is possible. Several chemicals can be used that will not hurt a given crop, but will destroy weedy plants. Examples are the use of monuron on asparagus plantings and simazine on sweet corn.
Pre-emergent Weed Killer and Crabgrass Control - how to use pre-emergent weed killer for controlling crabgrass.
Crabgrass Control - controlling crabgrass for a lawn free of the dreaded stuff.
What Does Crabgrass Look Like? - you can't control it if you can't recognise it.
Weed Killers - recommended chemical weed killers and the approach to using them.
Toxic Residue In Soil - it pays to be aware of not only the short term, but long term effects of weed killing products.
Where to next?
Learn more about the importance of soil testing and how to use a soil ph meter.
Discover effective tactics for killing weeds and in particular crabgrass control.
Or that essential ingredient of all successful gardens - read about building a compost heap or potash fertilizer.
Maybe you're a rose fan, in which case our rose gardening series will help with everything from growing miniature roses to rose trees.