 My father insists that water hyacinth and all eucalypti are weeds. I think pampas grass and fountain grass are weeds. Everyone has his or her own idea of what a weed is. Weeds can be trees, shrubs, vines, perennials, annuals or even aquatic plants. A weed is merely any unwanted plant.
The most common and prolific weeds are annuals and a few perennials. Less prolific tree and shrub weeds certainly get larger and can cause more damage eventually, but they are not as common and are generally easier to control--at least before they get large. Most weeds are not native, but have been imported either as ornamentals, to feed grazing livestock or to serve as lumber.
Giant reed that now infests the Sacramento River was actually used as packing material for Asian cargo unloaded at river ports in the 1850s.
Weeds are undesirable for various reasons. Many are merely unsightly. Many also compete with desirable plants for space, nutrients, sunlight and water. A few provide habitat for insect and fungal pathogens that may infest nearby plants. In large quantities, some weeds can be a fire hazard. Poison oak is undesirable for obvious reasons. Weeds like Russian thistle (tumbleweed) and foxtail, which have developed seeds that hitch rides on the hooves or in the fur of animals, can be dangerous to dogs and cats.
Herbaceous weeds--both annual and perennial--can be controlled with contact or pre-emergent herbicides. As the name implies, contact herbicides kill targeted weeds to which they are directly applied. Most are "translocated," which means they spread into all parts of the affected plants. Pre-emergent herbicides are applied to areas that were formerly infested with weeds to kill the next generation of seedlings as they germinate or to prevent seeds from germinating.
Selective herbicides will kill certain plants, but not others. For example, "grass killer" will kill grass weeds that have become intertwined with ground cover plants, without killing the ground cover.
Weeding by pulling or digging weeds out, or by hoeing or mowing, provides more immediate results. Mowing leaves the weeds in the ground, but interferes with their perpetuation by more or less removing the flowering or seeding upper portions. Hoeing also leaves the weeds, but hopefully chops and at least partially buries most of them so that they cannot recover.
Pulling weeds is certainly the most efficient method, but a few weeds can regenerate from even small pieces of root or bulb that might remain in the soil. Each of these methods is, of course, most effective if performed before the weeds have produced seed for the next generation. Mulching or planting dense ground cover inhibits new weed growth.
Flower of the Week:
Butterfly bush
While blooming during summer, butterfly bush, Buddleja davidii, is about as popular among butterflies as it is among those who plant it in their gardens. Butterflies enjoy snacking at the very small and slightly fragrant flowers, which are densely arranged on narrow conical flower spikes that are about 6 to 10 inches long. Flowers may be deep or pale purple, blue, pink or white. The narrow lanceolate (lance-shaped) leaves are variable, but are generally about 6 inches long with fuzzy gray undersides.
Butterfly bush may be almost evergreen during milder winters, or may freeze to the ground during colder winters. Cutting canes back to the ground at the end of winter, whether they are frozen or partially foliated, promotes vigorous new cane growth in spring. These new canes grow quickly to about 4 feet tall, and may get as tall as 8 feet. They are rather bare at the base and limber on top, arching with the weight of the flowers. Butterfly bush does not need much water when established, but looks better if watered somewhat regularly.
Horticulturist Tony Tomeo can be
contacted at 408.358.2574 or at LGHORTICULTURE@aol.com.
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