Back in the olden days, when I started gardening, herbicides were mechanical or mechano-organic, i.e., hoe and muscle. Over the decades many chemical herbicides have been developed. Good thing, too; my muscles were giving out.

Using herbicides is a little more complicated than using insecticides. Spray the bug with a bug killer and you kill the bug, right? Herbicides, though, come in different flavors that operate in different ways and on different plants. One type is called preemergent, and you’ll need that before long. Preemergent herbicides don’t kill weeds; they prevent them from growing in the first place.

Lawn weeds like crabgrass are annuals. In autumn they die after spreading seeds to germinate the next spring. You need to stop them with a lawn weed preventer. And since these don’t work once the seed sprouts, you need to do it early, about the time the forsythia blooms. That is unless you like weed grasses; some years they’re the only thing that’s green.

Other preemergent products, like Preen, are intended for use in flower borders. I sprinkle it on top of my bark mulch in spring for extra protection. But be careful. Don’t use it where you will be planting seeds later in the season.

For reasons I don’t understand, these don’t work on all seeds. For example, carrots seem to be immune. Now, most of us don’t have a big problem with carrot weeds in our perennial beds, but think for a minute. One of the most tedious tasks in gardening is weeding a new carrot patch. Since carrots aren’t affected, I put Preen down the day before I plant my carrots to eliminate most of that miserable chore.

Other chemical weed killers work on weeds that are growing already, but there are differences here, too. Some kill almost anything green, others are selective.

Broadleaf weed killers get rid of any weeds that aren’t grass. So it won’t kill your crabgrass, but it will get the dandelions without harming the lawn. Some people pull dandelions. And then pull them again. And again. Because if you leave one little bit of root -- and you will -- it grows back.

And some people put broadleaf weed killer over the whole lawn, usually in some combination product with fertilizer. I don’t like that either. Most of the lawn isn’t dandelions, so it is wasting a product I paid money for. I just keep a spray bottle handy and hit them one at a time. It is a good excuse to wander around aimlessly.

The WMD of herbicides are the broad spectrum formulas. They’ll kill just about anything green. And my favorite in that lot is Roundup. Yes, I have seen the TV ads claiming that Roundup is “linked” (a quibble word that is different from “causes”) to certain rare cancers. Organic types have been suing Monsanto for decades and finally found a jury that agreed with them, but the EPA still doesn’t.

I use Roundup to kill unwanted shrubs before I pull them out or a bad lawn before I reseed. I use it on the sidewalk cracks and the gravel driveway. I use it on the vegetable garden in early spring if the vigorous, mat-forming weeds get ahead of me. And I use it without any concern about harm to the environment. There is one thing you need to know about Roundup. It kills totally but not instantly. Things don’t die overnight, but their fate is nonetheless certain. In a few days they’ll yellow and wither away.

For gardeners who put huge effort into keeping plants alive, it seems perverse to spend money to kill them. 

But remember: Every plant you kill makes room for a new one you can go out and buy.