Marianne Ophardt

Ask 'why,' not just 'when'

When? It's a question many gardeners ask at this time of year.

I wish the questions were initiated with a "why" instead of "when" so gardeners could learn the science behind the answers.

When should I apply crabgrass "preventer" or pre-emergent? It would be better to ask, "Why should I wait to apply crabgrass pre-emergent until forsythia is in bloom?" Crabgrass seed germinates when soil temperatures reach about 60 degrees. That typically coincides with the blooming of the yellow flowering shrub forsythia.

If you apply certain pre-emergent materials early in the spring when the soil is still cold and crabgrass has no chance of germinating, the materials dissipate and are not as effective later.

Why do lawn care companies put pre-emergent control materials on earlier than you recommend? Commercial pesticide applicators have access to different materials than home gardeners. Many of the pre-emergent materials that lawn care companies use have a longer residual. Check the label on the product you use for information on how long it is effective.

It's important to point out that the "preventer" materials must be applied before the crabgrass seed germinates. Once a crabgrass seedling starts to grow, the preventer materials won't work.

When is it time to fertilize? More insightfully, "Why do you recommend waiting to fertilize my lawn until late April or early May?" I know how hard it is to wait. Spring is nearly here and everyone wants a green lawn. Me too. A brown lawn that is only green in the spots where dogs "fertilized" it during the winter is frustrating. Nevertheless, I have to be patient, because I know fertilizer applied this early will promote the lush green top growth that I want to see, but that this growth will be at the expense of the roots, turf density and overall lawn health. I want a nice thick lawn that's able to keep out invading weeds like crabgrass and dandelions.

When do I apply dormant sprays to my trees? "Why should I apply dormant sprays to my trees?" would be better. There are two major types of dormant sprays -- fungicides aimed at managing fungus disease and oils that target overwintering insects. The timing of dormant fungicides is specific to the disease being controlled and the materials being used for control. They're generally applied as a management tool when the plant has been infected in the past. They shouldn't be applied as preventatives because diseases can build up resistance.

Dormant oils primarily control aphids, mites and scale. The name "dormant oils" is misleading, though. The oils are really applied at the "delayed dormant" stage. That's just as the buds start to open. That's also when the insects are becoming active. Applied too early when the tree or shrub is dormant, the insects are dormant too.

When can I plant my frost-sensitive annuals? A more insightful question is, "Why should I wait to plant my frost-sensitive plants?" Our last average date of frost is between May 1 and May 15 in this region. Because this is an average, the last date of a killing spring frost could be in April or even June. (Yikes!)

Gardeners often like to get their plants in early, then utilize frost protection such as hot caps, Wall-of-Waters, tents and more. It makes some of us gardeners look like real procrastinators. Early planting coupled with frost protectio, only provides gardeners with a slightly earlier harvest. Cold soil temperatures and chilly days do not provide warm-season plants with the warmth they need. A week of cool temperatures, below 55 degrees, and cold soil, will stunt plant growth. That means procrastinating and waiting for warmer soil isn't so bad after all.

* Marianne C. Ophardt is a horticulturist for the Washington State University Extension Office in Benton County.