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Will Lawn Fertilizer Make Weeds Grow?

If you’ve fertilized your lawn before, you’ll already know what this amendment can do for your grass. Fertilizer is the key to making your lawn thick, green, and full of healthy thriving grass plants. This may lead you to wonder whether fertilizer also has an impact on the unwanted growth on your lawn; being that they’re plants, it’s logical to assume that lawn fertilizer could make weeds grow too.

In short, yes, lawn fertilizer will make weeds grow.  Fertilizer has the same effect on weeds as it does on grass plants; it delivers a boost of nutrients, in turn increasing the rate of foliar and root growth in the surrounding plants. It isn’t possible to kill weeds with regular fertilizer; you must first remove any existing weeds from your lawn before you fertilize it. Thereafter, once your lawn is weed-free, fertilizer plays a key role in preventing future weed growth. 

How Does Fertilizer Make Grass Grow?

Fertilizer is a substance that contains a range of nutrients, primarily nitrogen, phosphorus, and potassium, that grass needs to grow. Another way to think of fertilizer is as food for plants; when you add fertilizer to your lawn, you’re essentially feeding your grass.

a bag of granulated fertilizer spilling onto lawn

Like humans, grass and all other plants need a source of certain nutrients to survive. The most important of these nutrients are nitrogen (N), phosphorus (P), and potassium (K), these being known as primary nutrients. Additionally, plants need several secondary nutrients in lesser amounts, such as calcium, iron, and magnesium. Each nutrient serves different purposes within the plant; for instance, nitrogen drives foliar growth, potassium assists photosynthesis, and phosphorus encourages the growth of roots.

These nutrients must be readily available in the soil for plants like grass to take up. While soil naturally contains these nutrients, their abundance can deplete over time due to environmental influences like the soil type and water runoff. This is why it’s important to keep up with a regular lawn fertilization schedule throughout the year; adding fertilizer to your lawn ensures your grass always has enough nutrients to stay thick, healthy, and thriving.

Will Lawn Fertilizer Make Weeds Grow?

Yes, lawn fertilizer can make weeds grow alongside your grass. Weeds respond to fertilizer in the same way as any other plant would; they will experience a growth spurt upon taking up the fertilizer’s nutrients.

By definition, weeds are simply plants that grow in unwanted places; this is the only aspect that sets them apart from the rest of your wanted growth like your grass. So, being plants, weeds also take up nutrients from the soil to grow. Adding a layer of fertilizer will therefore provide any existing weeds in your lawn with a boost of nutrients. Consequently, the weeds will grow and spread more rapidly 

On top of this, most common lawn weeds are voracious growers with extensive root systems. With more roots to take up the fertilizer, weeds will steal the nutrients from your grass plants, before outcompeting them. Even if your lawn only has a few weeds to begin with, an addition of fertilizer can trigger a full infestation. 

Should You Kill Weeds Before Using Fertilizer?

You absolutely should kill weeds before using fertilizer. As we’ve just explained, fertilizer will only worsen any existing weed growth on your lawn. A few weeds can turn into a full infestation, while an existing infestation can completely outcompete the grass.

If you’re planning on fertilizing your lawn, but you’ve spotted some weed growth, you must deal with the weeds first. As an initial step, you need to identify the type of weeds growing on your lawn; correct identification is vital as different types of weeds require different methods of control. For instance, a weed killer designed for broadleaf weeds like dandelion or spurge will be ineffective on grassy weeds such as crabgrass or nutsedge. This is an important consideration when choosing the right chemical weed killer for your lawn. 

You must also ensure that the weed killer you choose is safe for use on grass. Non-selective herbicides like those containing glyphosate will kill all plant matter indiscriminately, including grass plants. These weed killers are only suitable for spot-treating weeds due to their capacity to kill the lawn. On the other hand, selective weed killers will only kill the weeds they’re designed for, leaving the surrounding grass unharmed. 

As an alternative to chemical products, there are also a few natural methods of lawn weed removal. For example, you could pull the weeds by hand if the infestation isn’t too severe. Take note that, in order for hand-pulling to be effective, you must remove the entirety of the weed and its roots; any part of the plant that remains in the soil can lead to a reinfestation. It’s also possible to kill weeds with salt or boiling water if you want to avoid using chemcial herbicides on your lawn.

weeds outgrowing grass on a lawn

Can Fertilizer Kill Weeds?

Regular fertilizer cannot kill established lawn weeds. Instead of killing the weeds, fertilizer will only help them to grow. With that said, there’s a type of fertilizer known as ‘weed and feed’ that works preventatively to kill weeds. 

Weed and feed is a combination of fertilizer and pre-emergent weed killer. In addition to enriching the soil with nutrients, this product also kills weed seeds before they germinate. As such, they’re an effective method of preventative weed control if you apply them at the right time. Just be aware that weed and feed products are ineffective on any young or established weeds post-germination. 

Also, in general, regular fertilization is an effective method of weed prevention for an already weed-free lawn. Using a high-quality fertilizer with the best NPK ratio for your lawn will strengthen it and increase its growth rate; the result is a thicker lawn full of strong grass plants that are naturally more resistant to weed growth. Again, this is why it’s crucial to keep up a regular fertilization schedule as part of your annual lawn maintenance.

How Does Fertilizer Help to Prevent Weeds?

Fertilizer helps to prevent weeds by reducing their favourable growing conditions, encouraging the growth of thicker grass, and promoting an early spring green-up. 

Makes Soil Hostile for Many Weeds

Using a high-nitrogen fertilizer on your lawn can make the soil more hostile to weed growth. Many types of common lawn weeds, such as dandelion and clover, thrive in poor-quality soils with a low nitrogen content. While the grass struggles to grow, these weeds take advantage of the poor conditions and take over the lawn. 

An application of a high-nitrogen fertilizer counteracts the favourable conditions for these invasive weeds. It simultaneously makes the soil less hospitable for weed growth, as the same time as strengthening the grass plants. As a note, take care when applying high-nitrogen fertilizers to your lawn; too much nitrogen can lead to fertilizer burn on the grass. Refer to the guidance on the packaging of your chosen product for its recommended application rate.

Encourages Growth of Thicker Grass

Fertilizer helps your lawn to outcompete weeds by thickening up the grass plants and strengthening their root systems. With thicker foliage and more robust roots, the grass plants will naturally withstand the growth of invasive lawn weeds. 

Weeds are opportunistic in nature, quickly taking over bare patches of soil and anywhere else they can squeeze in. If your lawn is full of patchy, struggling grass, it’s much more vulnerable to weed infestations. This is another reason why fertilization is so important; the nutrients in fertilizer encourage the grass to form a dense carpet leaving little space for weed growth. Any weed seeds that attempt to germinate in a well-fertilized lawn will struggle to take root, dying off soon after. 

Promotes Quicker Green Up in Spring

Keeping your grass well-fertilized to the end of its growing season will promote a quicker green-up in the spring. This will give your grass a head start on weeds that also germinate in the spring, like dandelions and crabgrass.

Grass stores extra nutrients in its roots to survive off of during its period of winter dormancy. In spring, the grass then uses these stored nutrients to green back up and return to life. Fertilizing your lawn ensures that your grass plants have plenty of nutrients ready for their spring green-up; and, the faster the grass gets growing, the less opportunity there is for weeds to take root. 

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