Chemical weed killers or herbicides should be used as sparingly as possible in gardens as a whole, but especially in private ones. Excessive use of them is bad for the ecological balance in the garden itself, as much wildlife is deterred from establishing itself, and in the wider sense, is a serious form of pollution.
Nonetheless, it is not easy to desist entirely from their application, although highly committed organic gardeners insist on doing so. As a devil’s advocate at least, one can outline the circumstances in which the use of herbicides is the most effective way of managing particularly troublesome weeds.
For example, virulent perennial weeds like Convolvulus, Cyperus, and Oxalis can totally ruin an herbaceous border, or a carpet of ground covers, unless eradicated prior to planting. Sometimes, the only way of getting on top of the perennial lawn grass that has crept into the flowerbed is to spray it with a selective, anti-grass weed killer.
It is even possible, paradoxically, to justify on ecological grounds, the chemical treatment of invasive alien species that potentially can devastate a natural habitat by replacing the local flora and in consequence, the fauna that is associated with it.
Yet whether you use herbicides with extreme reticence or otherwise, there are two types of weed killer that should be avoided altogether in small, private gardens. One is the pre-emergent category, which is sprayed or spread in granular form on the ground to deal with weed seeds that are about to germinate, while the other type is a group of herbicides that selectively kill broad-leaved plants, without damaging grasses and other monocots.
Other than environmental considerations, the application of either group often inflicts serious damage to garden plants in the vicinity, including those in neighboring gardens.
The problem with the pre-emergent weed killers is that they contain residual properties, remaining active in the soil’s top layer for a certain period of time. If the amount applied is excessive relative to the area treated, then either the soil or neighboring plants may suffer.
They are actually less dangerous when used in large open spaces because at least the application rate can be easily calibrated. On the other hand, the granular forms that are generally recommended for spreading over small spaces, are far more difficult to calibrate accurately. For herbaceous beds in private gardens, it is preferable from every point of view, to reduce weed germination by means of organic mulch.
The selective herbicides that are applied to eliminate broadleaved plants are derivatives of the dreaded 2-4 D. They operate by disrupting the hormonal balance of the plant. They are dangerous because they evaporate very rapidly, resulting in vapors containing the poison landing on garden plants.