A guy last week asked me about weed killer. Twenty minutes later, I think I explained it, but he was bleary eyed. The two-second answer is “it depends.”

A guy last week asked me about weed killer. Twenty minutes later, I think I explained it, but he was bleary eyed.

The two-second answer is “it depends.” Here’s a distillation of our conversation. Each class of weed requires a different approach.

GET TOGETHER

The best way to fight weeds is to get your neighbors involved. All it takes is one yard with dandelions to infect everybody else. If everybody’s on the same page, you’ll improve the whole neighborhood.

PRE-EMERGENT HERBICIDE:

Wait until mid-April to May to apply. These generally are effective for about six weeks. Apply too early and you will miss the window for protection this summer.

Many lawn fertilizers contain pre-emergent fertilizer. It’s usually more effective to fertilize first and then apply the herbicide when the grass is fully growing. This is more expensive but can be worth it.

DANDELIONS

How pretty, but what a nuisance. They are so invasive, they grow in cracks of the patio. Then they bloom, sending airborne seeds for blocks.

Despite all the science applied here, the best way to get rid of the No. 1 weed problem is to pull them, taproot and all, before they go to seed.

Dandelions thrive in nutrient-unbalanced, calcium-deficient soil. The best prevention is to increase the nutrition with an organic fertilizer.

If you go chemical, use a liquid weed killer after the crop develops but before they go to seed. You’ll only have a few days here, hopefully not rainy ones.

BROADLEAF

We fight more than 200 varieties, the most common being foxtail, chickweed, mallow and ground ivy, plantain, sorrel and violets.

The best prevention is the same with dandelions. A healthy yard will, over time, allow your grass to choke out the weeds. Mowing your yard at the highest setting promotes grass growth.

Spray weeds on a dry day, and do not water for three days. You may need to spray again. Depending on your outbreak, it’s better (and cheaper) to fight broadleaf with a spot-spray bottle than spreading the whole yard with a chemical killer.

WEED PREVENTERS

These products are based on corn gluten, which blocks the formation of roots of all types. Preen is the most advertised.

You’ll need a high concentration of product to accomplish this, up to 80 pounds per 1,000 square feet with a followup later this summer. Weeds controlled include dandelion, bentgrass, purslane, foxtail and Bermuda grass.

Corn gluten is best applied in spring and fall, but you must time it. It stops new weeds but will not kill existing ones. It also stops the good guys, the unborn grass seeds. It must be watered after application.

One application is effective for four to six weeks. Be careful. It will halt growth of your garden seeds.

CRABGRASS

This is a tough guy. Late in the summer, each plant releases hundreds of seeds that can travel for miles on the wind. Liquid herbicides labeled for crabgrass are needed once the plants emerge. Time your spraying before seeds are formed.

Expect to spray multiple times. Pre-emergent crabgrass herbicide is applied to stop plant germination. Once again, timing is important. Applying it too early will leave your lawn exposed during the primary crabgrass-growing season in mid-summer. Check the product label for advice.