Questions about Roundup and Horticultural Vinegar

Dear Bay Weekly:
    I have a question about The Bay Gardener’s Guide to Spring, published in the March 31 Home and Garden Guide. Under the heading Preparing Garden Soil, Dr. Gouin writes that when preparing the garden bed, “to conserve soil moisture and energy, simply hoe out the weeds or kill them with either horticultural vinegar or Roundup about a week before you intend to plant.”
    At my local nursery, I was told that I would need to wait at least a month after applying Roundup. Now I’m at al loss. What should I do?

–Delores Pennington, St. Leonard


The Bay Gardener Clarifies

    Roundup, chemical name glyphosate, is not applied on the soil but on the foliage, thence translocated down to the roots. That is how it kills plants. It takes four days for the glyphosate to translocate down to the root tips.
    By label, Roundup requires 10 days between application and planting. The point of the 10-day pre-planting period is to give the chemical time to reach and kill the roots of perennial weeds like dock, dandelion and brambles. Our studies showed that in fact only four days are needed for the translocation of the chemical through the soil.
    The waiting time is not to prevent the effects of roundup on the soil or in garden. When Roundup gets into soil, it’s deactivated by colloids and bacteria that break it down and release phosphorus. The effects of Roundup on the plants you intend to eat is not a problem.
    For annual (as opposed to perennial) weeds, you’re better off using horticultural vinegar, because when you kill the top, you kill the root.
    Horticultural vinegar (20 percent concentration) was approved for all garden use in Maryland two years ago, and up-to-date nursery centers might carry it. Until recently, it had to be ordered. I order mine online from A.M. Leonard (www.amleo).