Strategies For Fighting Weeds In New Topsoil

Q: The topsoil I ordered seems to be full of sprouting weeds. What can I do?

A: Landscaping changes frequently require additional planting medium . . . to fill new raised beds, berm up an inhospitable compacted soil, or fill in behind newly terraced areas. What you get when you order a "topsoil" mix can vary, since there currently are no legal standards for uniformity of mixes.

Topsoil, as such, is a rare commodity, since it is the product of slow geologic weathering over thousands of years, plus the decomposed vegetable matter that's fallen into it. The presence of millions of tons of glacial ice over the Puget Sound area 12,000 to 14,000 years ago has left many of our potential gardens compacted masses of clays, hardpan and rocks. Often, ordering from a landscape-supply company may be the best option.

The most commonly available mixes are ordinarily called 3-way or 5-way mixes, and they contain various proportions of subsoils, sand, sawdusts, manures and composted-sewage sludge from municipal waste.

While these mixes are convenient, several problems may occur. First, the sawdust may be insufficiently decomposed and may result in a nitrogen tie-up in the mix that results in plants growing with yellow, chlorotic leaves, or growing somewhat stunted. A good, quick way to test a newly spread mix for this problem is to scatter some radish seeds: If they emerge as a good solid green with healthy, sturdy leaves, you may have no nitrogen deficiency in the mix. (And they come up so fast that you will have a fast, rough idea.) Add nitrogen fertilizer (such as a grass fertilizer without any herbicides) to assist in the first-year sawdust breakdown.

Another possible difficulty is sources of soluble salts, which retard the growth of new plants; let rain or irrigation leach the mix out for several weeks before planting. And you may indeed experience weed breakouts, either from weed seeds in manures or from weeds in the soils mixed into what you purchased.

Deal with these before planting; use cultivation to remove seedlings before they get established. (Hoe, hoe, hoe.) Two to 3 inches of mulch applied to the surface after you plant will also help lessen weed-seed germination.

Look carefully at mixes before delivery; check for chunks of roots from weeds, for large bits of undecomposed wood or other foreign matter. Some mixes may be the equivalent of letting the Trojan Horse, with all its enemy warriors, into your garden.

Q: Are there any roses especially suited to the maritime Northwest?

A: Anyone who's grown roses knows that mildew, black spot and other diseases can attack all too often in Western Washington. Several handsome disease-resistant roses are listed on WSU/King County Cooperative Extension's publication "Roses" (KC 107).

Choose "Honor" or "Just Joey" or "Peace" for white, oranges or yellows in hybrid teas, and Queen Elizabeth (pink) or Iceberg (white) in grandiflora and floribunda. While no modern rose is immune to disease or insect problems, these and others listed on KC 107 may be sturdier garden subjects. Also, try the older shrub roses, many of which exhibit considerable disease resistance.

To receive the publication, send a stamped self-addressed envelope requesting it to Cooperative Extension, 612 Smith Tower, Seattle 98104.

Gardening runs Friday in Scene and Sunday in Home/Real Estate. It is prepared by George Pinyuh and Holly Kennell, Washington State University/King County Cooperative Extension agents, Mary Robson, Master Gardener program assistant, and volunteer Master Gardeners. ----------------------------

GARDENING QUESTIONS?

Have a gardening question? You can now reach us 24 hours a day by dialing 464-8470 and recording your question. Or, write: Gardening, c/o Seattle Times, PO Box 70, Seattle, WA 98111. Questions of general interest are answered as space allows in the Gardening column, which runs Friday in Scene and Sunday in Home/Real Estate.

If you need immediate assistance, Washington State University/ King County Cooperative Extension suggests calling a Master Gardener at 296-3440 weekdays between 10 a.m. and 4 p.m.

Or, tap into the Dial-Extension tape library of more than 200 titles. Call 296-3425 for a free catalog.