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Can Fertilizer Kill Weeds | What You Should Know

Lawn with Many Weeds

Last year I was working on rejuvenating my lawn and read online that some fertilizers actually kill weeds. I thought this notion was ridiculous but decided to look into the matter. I did some research and made some calls, and this is what I learned.

Fertilizer can kill weeds if an excessive amount is applied. Unfortunately, that much fertilizer will likely kill the plants you are trying to cultivate as well. With normal use, regular fertilizers will not usually kill weeds but boost their growth instead since they are technically plants as well.

My conclusion is that whoever wrote the information I read was either talking specifically about specific types of weeds or about “feed and kill” fertilizers. I’ll cover those first but also talk about high-nitrogen fertilizers as well as when and how to apply weed killer.

Feed and Kill Fertilizers

There are brands of fertilizer that are designed to grow plants while killing weeds at the same time. These “feed and kill” ones are usually formulated with specific types of plant poison that are supposed to kill weeds only and not do damage to anything else. However, things don’t always work out that way.

In practice, the herbicides in these formulas are distributed unevenly and are known to kill whole patches of grass. They are also often filled with synthetic fertilizers that can compromise the soil’s well-being in the long run.

To top it all off, they are not that good for the environment, and the chemicals in them are very likely to end up in your home as well.

All in all, it’s better to do two separate applications, weed killer, then fertilizer.

Will High Nitrogen Fertilizer Kill Weeds?

High-nitrogen fertilizers may kill certain types of weeds. The smaller weed species that usually hide in your grass, like clover or dandelions, are not big fans of nitrogen. On the other hand, chickweed, amaranth, henbit, and chicory thrive when nitrogen is applied.

Actually, seeing particular species of weeds can tell you what’s going on with your soil. For example, the already mentioned dandelions indicate a lot of potassium but very little calcium in the soil.

Low calcium levels can also be a cause of another weed that usually pesters lawns – crabgrass. Groundsel, knapweed, and ostrich fern are signs of excellent and fertile soil.

But be worried if you see moss or ragweed since they indicate poorly nourished soil and low fertility.

What Is a Good Nitrogen Fertilizer?

You’re in luck; there are quite a few excellent ones on the market. Milorganite is my number-one choice and has been a favorite of many professional landscapers since 1926. It’s also considered one of the most amateur-friendly fertilizers currently available.

Milorganite releases slowly, and it’s supposed to provide nutrients for up to 10 weeks after fertilization. You can use it on your lawn, flowers, vegetable patch, etc.

It also comes with an extra shot of iron that makes lawns super green and vibrant. But keep in mind that some pros recommend you don’t use it alone but alternate it with another fertilizer for best results.

You can pick Milorganite up at any garden store or just order it off Amazon. (Click the link to see the listing)

Can You DIY a Good Nitrogen Fertilizer?

Compost has relatively low nitrogen levels when compared to the store-bought stuff. But you can increase them slightly if you add more coffee grounds and tea leaves to your recipe.

Should You Fertilize or Spray for Weeds First?

Always spray for weeds first. Weed killers will affect the soil, so you would probably have to go back in and fertilize it again.

Pro Tip: Unless the directions tell you otherwise, water the area first, then apply the herbicide. After that, it’s a good time to perform a health check on the soil. There should not be any issues, but it’s better to be safe than sorry.

Look for signs of other life, like mushrooms or worms. Healthy soil also tends to be darker and crumbles easily. If everything checks out, you can proceed with your preferred fertilizer.

You can fertilize the soil every couple of months, but there should be no need to apply a herbicide more than once a year.

What Is a Good Weed Killer?

Unfortunately, we don’t have one Holy Grail product that can kill all kinds of weeds. In short, anything with the potential to be that effective would probably destroy all other plants as well.

If you are the average homeowner, you are most likely dealing with crabgrass and dandelions that are ruining your lawn. They are incredibly annoying because you can’t just pull them out by the roots since they are small, and there are often a lot of them.

Currently, your best bet is Ortho Weed B Gon (Click the link to see the exact type I use).

The reason why this product is not worthy of the Holy Grail title is that it comes with a set of very specific instructions. If you follow them to a T, you will get rid of all the weeds without hurting your lawn. If you don’t, you’ve just wasted your time.

However, if the situation is bad enough that you need something more potent than that, you better leave the job to the pros. After all, weed killers are plant poisons, and it takes a small miscalculation for things to go terribly wrong.

Bottom Line

In the end, my lawn ended up making a huge comeback. While I did end up with a few weeds here and there, the fact that all the bare spots were gone and the grass itself was really thick was sort of a miracle.

The nitrogen fertilizer I applied early in the year made the weeds go crazy, as you can see below. It definitely didn’t kill them!. They also ended up being fairly resistant to weed killers. So, I basically had to just firmly rake them up or pull them up by hand. It took forever, but it was worth it!

I hope this article has been helpful. Thanks for reading!

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