Getting it right: pre-emergent herbicides and fertilizer

By Bonnie Orr
WSU Chelan/Douglas County Master Gardener

Man in red shirt using hand held sprayer.
It is time to apply pre-emergent herbicides. Many people mistakenly believe that compost is fertilizer. Compost has very low fertility — usually less than 3%. – Getty Images/Unsplash
Bonnie Orr
Bonnie Orr – WSU Extension Chelan/Douglas County Master Gardener – photo by Don Seabrook, Wenatchee World

It is time to apply pre-emergent herbicides. Pre-emergents are products that are spread on the lawn to prevent germinating seeds from growing. In lawns, crabgrass is one of the primary targets.

The timing of the application is essential. In our area, pre-emergent is applied when the forsythia blooms or when the redbud tree is just beginning to bloom. These are indicators that the soil is 50 degrees at 2 inches depth. Forty-five days later, another application can be applied at half-strength if your lawn is spotty and not very vigorous. Lawn weed seeds grow best where the turf is thin and sunlight can reach the soil. Healthy turf is the best way to get rid of crabgrass.

Pre-emergents for spotted spurge and purslane are applied later because those seeds germinate when the soil is about 65. WSU AgWeathernet ( will tell you the soil temperature in your area. You can set up a free account.

Why do you need a fertilizer? Have you ever had a soil fertility test done on your soil? How do you know what amendments you need to add? Often people bring their soil test results for Master Gardeners to advise and interpret. And many times, the soil has an excess of either N, P or K — Nitrogen, Phosphorus or Potassium. N, P and K are the most common macronutrients in a bag of either lawn or garden fertilizer. Our native soils in Eastern Washington contain sufficient phosphorous. Excess amounts of phosphorous or nitrogen can cause as many growing problems as low levels of fertility in addition to contaminating the ground water and the rivers. You are wasting money buying and spreading product that the soil does not need. It would be a good idea to get a soil test done before this year’s gardening season starts.

Do you use organic or conventional fertilizer? The bottom line is that the plant does not know the difference. Organic nitrogen or synthetic nitrogen is going to provide the same benefits to the plant. Nitrogen is Nitrogen — the same with other supplements. A gardener makes the choice based on how he or she wants to deal with the soil and with the fertilizer residues in the soil.

Using fertilizers seems to be problematic. The adage “more is better” is truthfully detrimental to your plants and to your soil. Read the directions on the package. These directions are not merely suggestions; directions are based on the company’s research before marketing their product.

Many people mistakenly believe that compost is fertilizer. Compost has very low fertility — usually less than 3%. Compost’s function is to help the soil retain water, create more loft in heavy soils and provide insulation from high or low heat. Worms and other organisms break down the compost during the growing season. Research varies on the volume of compost to add to the soil. The percentage varies from 5% to 15%. It is such a little amount that it does not make much of a visual difference in the soil. Too much compost will hinder plants’ growth.

Organic supplements such as rabbit, llama, chicken or cow manure must be totally rotted down before being added to the garden for two reasons. The first reason is the amount of salts in the manure that can accumulate in the soil. The second reason is that some manures are very high in nitrogen and will negatively affect the growth of your garden. The two most common problems are huge, lush plants and no blooms or fruit production and blossom end rot in tomatoes and peppers.

A WSU Chelan and Douglas County Master Gardener column appears weekly in The Wenatchee World. To learn more, visit or call (509) 667-6540.