1 Mechanical digestion is evident, where one can see the breaking down of large food particles into smaller ones after being ground by the teeth. 2  While the chemical changes that take place happen within the body and are not visible.
Using a pre-emergent herbicide around your property could stop these territorial plants from overtaking your home's most valued green space. Pre-emergent herbicide is a type of weed killer that prevents weed seeds from sprouting. Unlike normal herbicide, it is meant to stop weeds from growing in the first place by spraying or spreading a chemical onto the soil so that weeds have difficulty placing down their roots.
Preen isn't a weed killer per se -- it's a pre-emergent which means it prevents weed seeds from germinating. It won't do anything for the weeds that are currently in your beds. I have on and off used Preen in my gardens.es, as Dorothy said, Preen is not a weed-killer, it's a pre-emergent and its effect lasts for about 3 months, not more. Whatever weeds are already in a soil they will grow thru it, but it will prevent FRESH seeds from germinating.