Water plants are any type of plant life that requires water to live and thrive. Some water plants secure their roots into the muddy soil under the water, while others float freely in the water with their roots collecting nutrients wherever the current takes it. Normally these plants make an excellent addition to any pond or lake, but occasionally they grow out of control, blocking out sunlight and affecting the marine wildlife in the process. When water plants become a problem, it's time to find a means of destroying them.
Determine the necessary type of herbicide. Systematic and non-selective herbicides will kill an entire plant when applied correctly--and sometimes even all plants in the area. Contact herbicides will cause the plants to die back, but leave the roots intact for later growth. This approach is better if the problem is simply a garden that has grown out of control. Selective herbicides are specialized treatments that target only particular plants are do not affect any others.
Determine the particular state's requirements for using herbicides. Some states (such as Washington) require that those applying herbicides to large lakes obtain a license from the Department of Agriculture.
Determine the means of applying the herbicide. Some are available in a liquid form the be sprayed onto the plants directly, whereas others come in a pellet form and are meant for dropping directly into the water.
Decide upon the best time to apply the treatments. Plants known as emergent vegetation, such as cattails, die off best when the herbicide is applied as the plants are sending out their seeds, because they also obtain larger amounts of root nourishment at that time--whereas plants known as submerged vegetation should be treated in May or June before their maximum growth is achieved.