Digging In: Advice on Weed Seeds in Straw Mulch, Climbing Hydrangeas

By Scott Aker
Special to The Washington Post
Thursday, September 10, 2009

Q I reseeded my yard last fall and used a straw mulch. I now have a healthy crop of "hay" growing in the lawn. Can I use an herbicide that would selectively kill the hay? If not, will it die off this fall?

A If you used straw, it probably came from a wheat or barley field. Most of the seeds in the straw are likely to be the loose grain that was not harvested, and since those grains are annuals, the problem will not persist beyond this growing season.

Straw may contain other weed seeds. Yellow foxtail, prickly lettuce and horseweed are just a few that may sneak into your garden or lawn in straw used as mulch. Fortunately, most of the weeds found in straw will decline or disappear with frequent mowing, so you can usually avoid deploying herbicides to eliminate them. You can reduce the number of viable seeds in straw by watering the bales thoroughly to encourage seed germination. When the straw is spread as a mulch, the weeds will die. Straw can take up enormous amounts of water, and it may take a month or more for wet bales to dry, so you will have to plan ahead if you take this approach.

You mention hay, which is an entirely different product. Hay is a mixture of grasses and broadleaf plants that is cut for use as fodder for horses and other animals. Straw has comparatively low food value and is usually used for bedding, not fodder.

Because it has the mature seed heads of a wide variety of plants, hay is a potent source of weed seeds. It should be used in the garden only if it is thoroughly decayed. Sometimes you can get spoiled hay, where microbes force a spike in bale temperatures and kill weed seeds. This may be used as a mulch in the vegetable garden, vineyard or perennial border before it has fully decayed.

Four years ago, I planted two climbing hydrangeas that have failed to bloom. What can I do?

You may have to wait a few more years. Climbing hydrangea, Hydrangea anomala subspecies petiolaris, grows slowly at first and may take six years to reach flowering size. Growth is slower in dry conditions, so water the vines well in times of drought. Avoid pruning them, since they may take a break from flowering in response to heavy pruning.

The very similar Japanese hydrangea vine, Schizophragma hydrangeoides, is a bit more precocious in its flowering, and the cultivar Moonlight has leaves that are shaded with silver, making it very attractive even when not in bloom.

Scott Aker is a horticulturist at the U.S. National Arboretum.

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