Scotch Broom, Scourge of the Northwest

There is a class of weeds that are legally defined as NOXIOUS. This is a special term applied to weeds that are so aggressive as to be considered economic and ecological dangers. They can completely alter an ecosystem by changing the soil nutrients, pH, or forming a monoculture that excludes other plants. Dandelions may be weeds (a weed control specialist once termed them obnoxious), but they are not designated as noxious because they are not capable of destroying crop value or ecosystems. They and many others are “background weeds” we live with.

The people who decide who gets noxious weeds status are the state and county noxious weed control boards. The Department of Agriculture is the supporting agency for these boards, because noxious weeds were recognized as an economic problem for agriculture. Controlling noxious weeds also protects ecosystem integrity in natural areas. Many weeds that don’t seem that bad to the average person can be very bad when they are unleashed in the countryside.

Scotch broom (or Scots broom) is a noxious weed. Its natural enemies are back in the homeland (Europe) so it can spread at will here. However some insects that eat seeds and vegetative parts of it have been introduced and may help control it.

We are dealing with the legacy of soil disturbance from grazing, logging, and trail blazing followed by Scotch broom invasion. When our restoration work was done, huge patches of it were mowed, and mowed again in subsequent years. But mostly just keeps the plants short. To kill it, one can cut large stems during summer drought, spray when it’s in bloom (highly effective), or pull small ones. Each method has its advantages and disadvantages. In an area where the seed has fallen and built up in the soil, it can take many decades to exhaust the seed bank. Pulling can bring seeds up to the surface and result in a whole new crop the following year.

Which brings me to my latest small project: due to weather and other circumstances, a couple of our scotch broom patches had not been mowed for one or two years, and it had grown taller. This actually makes is easier to spray, since when it’s short it is hard to kill because it doesn’t have enough leaf area and doesn’t bloom.

This winter I tried using a dormant spray on the evergreen stems which were more ‘alive’ due to the mild winter we had. It is more difficult to get all stems covered with a dormant spray, but easier to avoid killing the plants you want, as they have all died back. A combination of Triclopyr (Element 4A), and an oil/adjuvant (Mor-Act) seemed to do the trick, although it takes a heap-o-spray to get total coverage. I sprayed again during bloom, which was very effective.

IMG_0705

invaded area – previously sprayed broom is dead and dying. blooming plants are overtopping wildflowers in the nectar plots

IMG_0704-2One problem area was the nectar plots where we had planted wildflowers. The broom had invaded them over the years. Suddenly after a couple of years of no mowing, it was blooming. I couldn’t spray it without killing the wildflowers.

I hauled the weed wrench up the hill and brought along my spray bottle of herbicide and loppers. The ground was still soft from recent rain, so I thought perhaps I could minimize disturbing the seed bank by careful use of the weed wrench to pull the broom. It’s a satisfying plant for pulling – the taproot comes easily out of the ground with a ripping sound and not much fuss. But this was a rather slow operation. Actually weed wrenches are often more attractive as an idea than a tool. They are fun and easy to use, but the soil disturbance frequently outweighs their benefit.

IMG_0706

I thought perhaps I could test the effectiveness of various methods, and speed things up too, so I switched to cutting with loppers, spraying the ends immediately with a 1:3 mixture of triclopyr (Garlon) and water from a spray bottle with a little surfactant – a variation on the “cut and paint” method. The weather was hot, the broom plants numerous, and so I decided to also add a third treatment – just cutting. I am curious about how late in the season cutting alone will kill broom. Ideally, that’s a method for older large stems that have more trouble resprouting and it is supposed to be done in the hot dry summer to make it doubly difficult. Some of the stems I was cutting were smaller, and we subsequently had several weeks of cold damp rainy weather, so I don’t expect those to disappear. However, at least the wildflowers had some light to grow.

done at last

done at last

Same plot three weeks later - rose checkermallow in bloom. Others in foreground still struggling

same plot three weeks later – rose checkermallow in bloom. dead broom mulch

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