No discussion of the pests affecting ornamental plants is complete without considering the weeds. A weed may be defined as a plant hav­ing no positive economic value and/or growing in a place where it is not desired. Unlike insects and pathogens, weeds do not derive their sustenance directly from other plants as parasites. Instead, weeds com­pete with other plants for the materials both need to grow and thrive. In addition, weeds often serve as alternate hosts, providing sites for the overwintering of insects or pathogenic inoculum. Some fungi need weed species in which to produce one or more spore forms as part of a complex life cycle. In summation, weeds affect ornamental plants adversely in the following ways:

• as competitors for space, nutrients, water, light, heat energy, and CO2

• by shading crop plants, further inhibiting their growth

• as alternate hosts for insects and pathogens

• by crowding due to excessive vegetation and by the vining manner of some weeds

• through infestation of the soil with large quantities of seeds and root stock that permit proliferation of the weeds over many seasons

• by production of chemical exudates that inhibit germination and/or normal growth of crop plants