What is the best time of year to use Casoron and/or Preen for weed control on ornamental beds?
Both of these herbicides are registered pesticides, and the law requires that they be used in strict accordance with the directions (and only on the weeds/pests for which they are registered). It is safer for you and the environment if you manage weed problems without the use of pesticides.
You may wish to know more about these particular pesticides. Both Casoron and Preen are pre-emergents, meaning that they work to kill seedlings before they sprout. This means they will not eliminate weeds that have already broken through the soil surface and are growing above ground.
Casoron is persistent in both soil and water (i.e., it hangs around). Its active ingredient is dichlobenil. There are numerous environmental and health concerns associated with this chemical. Dichlobenil will kill any plants which are exposed to it, and will harm beneficial soil microorganisms. Below is a fact sheet about dichlobenil from Northwest Coalition for Alternatives to Pesticides.
The active ingredient in Preen is trifluralin. It is a suspected carcinogen, and is toxic to fish and aquatic life, and earthworms. Here is more information from Cornell University(now archived) and Extension Toxicology Network UK.
The links below provide information about alternatives to chemicals for weed control. Northwest Coalition for Alternatives to Pesticides has a page of factsheets about specific weeds and ways to manage them. Here is their page on managing weeds in garden beds.
Before reaching for chemical weed control, it makes sense to adopt gardening practices which will help keep the weed population low. Mulch is an excellent way to control garden weeds. After you manually remove weeds from an area of your garden, apply a layer of mulch. This should suppress weed growth and help retain soil moisture. Here is what garden expert Cass Turnbull says about mulch:
“Not only does mulch retain water, smother tiny weeds and weed seeds, and make it easy to pull new weeds, it is also harder for new wind-borne weed seeds to get a foothold.
“Mulch can be spread anywhere from 1 inch to 4 inches thick. The thicker it is, the more effective and longer lasting. Spread it thick in big empty spaces. Spread it thin around the root zones of shrubs to allow for sufficient air exchange, especially around shallow-rooted plants like azaleas and rhododendrons. And never let mulch stay mounded up in the base or the “crown” of a plant. It can cause crown rot on some shrubs and can kill them, even a year or more later.”
Source: The Complete Guide to Landscape Design, Renovation, and Maintenance, Betterway Publications,1991.