- Some herbicides specifically target weed seeds. These types of herbicides are known as pre emergent herbicides. These types of herbicides are applied throughout the spring on a monthly basis. Pre-emergent herbicide works against weeds, such as crabgrass, by preventing the seed from sprouting. It may be used in grass because the grass plants are already sprouted. Pre-emergent herbicide will not harm established woody plants such as trees and shrubs.
- Broad-spectrum herbicides are commonly sold as spot treatment for weeds in garden centers. These herbicides kill all plants that they come in contact with, including trees and shrubs. A broad-spectrum herbicide may not entirely kill a tree or shrub if a low dose of herbicide is used and the tree or shrub is very large, but it will weaken the plant. Weakened plants that aren't completely killed may be open to damage from insects or fungus.
- Selective herbicides are formulated for use with specific types of plants. Selective herbicides may kill one type of plant and leave another alone. Broadleaf herbicides are an example of selective herbicides that are used on grass because they may be sprayed over grass and weeds indiscriminately. Broadleaf herbicides will kill the weeds but leave grass alone. Because weed and trees or shrubs have similar plant structures, broadleaf herbicides will often kill trees and shrubs along with weeds.
Weed and Feed
- Some chemicals use a fertilizer as a carrier for the herbicide, these fertilizers, which are known as weed-and-feed products are typically used when weeds are uniformly distributed over an entire lawn. They should not be considered unless weeds reach epidemic proportions. Weeds that only infest part of a lawn may be treated using alternative means. Weed-and-feed products may cause ornamental plants, such as trees or shrubs, to absorb the chemical herbicide. This can cause damage in the woody plants.