Weed Killer or Fertilizer First? (Which one to Put Down First)

Weeds need the same things lawn grass needs to thrive, i.e. sunlight, water, and nutrients. Unfortunately, weeds can still thrive even when the grass is struggling from soil compaction, lack of enough nutrients, or less water. 

Most weeds are deep-rooted and grow quickly, and they will choke lawn grass if left on the lawn and therefore should be eliminated for the grass to thrive. But should you kill the weeds or fertilize the lawn first? 

As a thumb rule, you should kill weeds first, and then wait for 10 – 14 days before fertilizing the lawn.

Weed Killer or Fertilizer First?

Weeds are the number one enemy in achieving a lush green lawn. They are invasive and will compete for nutrients, water, and sunlight with your lawn grass. This will result in the grass thinning or dying. 

To achieve a manicured dense turf, you will need to feed the grass and kill the weeds. But the order you use will determine the success of the exercise.

Kill weeds first

Although healthy weeds absorb herbicide better, feeding a lawn infested with weeds will limit the nutrients absorbed by the grass as the weeds will also absorb some. Therefore, you should kill weeds first and then wait 10-14 days before fertilizing.

To grow a dense turf that can choke weeds, you will also need to aerate and overseed. You will need to remove the weeds and then prepare the bare patches for overseeding. 

To kill weeds, you can use several methods such as;

1. Spot treating with a herbicide 

Spot treating is very effective for weeds growing in patches and not when they are spread across the lawn. It works better because it won’t stress the turf as you only spray sections with weeds. 

Always use the herbicide as directed by the manufacturer. Non-selective weed killers may also kill your grass if incorrectly applied.

2. Post-emergent herbicide 

A pre-emergent herbicide is only effective in preventing weeds from sprouting. An example is a pre-emergent crabgrass preventer that kills the weed seedlings before they emerge from the soil. This approach needs proper timing.

3. Vinegar 

Vinegar is a natural method of killing weeds. It is acidic and will burn any plant it comes into contact with, killing it within 24 hours. To make a weed-killing formulation, you should mix it with dishwashing soap and then spot-treat the weeds.

Check here, how to use vinegar as a weed killer.

4. Baking soda 

Alternatively, you can also pour some baking soda on the weeds, but you need to sprinkle some water on the weeds first. It is also a natural method of killing weeds but will take several days before you see results. Avoid sprinkling it on lawn grass as it will kill it. 

Next, Add Fertilizer 

After putting weed killer, you should wait 10-14 days before applying fertilizer. Some weeds are tough to kill, and applying fertilizer immediately will work in their favor.

A few days after applying the weed killer, you should check the lawn to see if the weeds are dying because some might go dormant and not die. 

If you have some remaining, you might have to dig them out, or spot-treat again. But if they are dead, wait for at least 10 days after weed-killer application to fertilize. Check the best time to fertilize a lawn before or after rain.

Fertilizing after all the weeds have died is the most satisfying as the grass will absorb the nutrients with no stress or competition. If you have bare patches, this would also be a great time to overseed.

When should I Put Fertilizer and Weed Killer on my Lawn?

Timing is very important when it comes to fertilizing your lawn or using weed killer. They should be applied when the grass is actively growing because stressed grass can thin or die.

Pre-emergent herbicides should be applied before weed seeds begin sprouting. The most effective time is early spring and fall. However, since climate varies in different regions, apply when the soil temperature is around 55°F.

For post-emergent herbicides, you should apply them whenever you see weeds on your lawn. However, they are more effective when the weeds are small and actively growing as they absorb herbicides better. They should be applied between early to late spring when the soil temperatures are between 50-55°F.

Avoid mowing right before applying weed killer, as mowing stresses the grass and weeds. Stressed weeds will struggle to absorb the herbicide, meaning you may not achieve the desired results.

Also, wait for at least 4 days after applying weed killer to mow the grass because mowing sooner will remove the herbicide before it is absorbed and translocated to the roots.

Deep watering after the application or heavy rainfall will wash away the herbicide minimizing its effectiveness. It’s better to water the lawn a day or two before application to enhance absorption.

It’s best to apply the herbicide early in the morning before the temperatures rise because plants are actively growing in the morning more than any other time of the day.

Limit applying herbicide to two times a year as the regular application will affect the pH of your soil. You can also choose to use other natural methods of controlling weeds that are more environmentally friendly such as; pulling the weeds out, using corn gluten, baking soda, or vinegar.

Can you Mix Weed Killer and Fertilizer Together?

Mixing weed killer and fertilizer seems like a good idea as you will kill two birds with one stone. Fortunately, there are many options for “Weed and Feed” formulation in the market.

Although this combination saves time, energy, and money, it still has some shortcomings, such as;

  1. It is not ideal for a lawn with patches of weed – Since you must spread it to the whole yard to feed the grass, you will also be spreading the weed killer to the entire yard, which is unnecessary.
  2. Most Weed and Feed formulas contain herbicides for broadleaf types, and applying them on a lawn with flowers, shrubs, and trees might kill them.
  3. Applying the Weed and Feed might be ineffective as weed killing and fertilizing the lawn should happen at different times of the year. 

Weed and Feed are perfect for a grass-only lawn but should be used sparingly. You should only use weed killers at most twice a year and feed your lawn at most five times a year. 

Instead, you should use separate weed killer and fertilizer. This way, you can test your soil and put the fertilizer with the right amount of nutrients needed by your lawn which is not an option for Weed and Feed. 

Also, if you do not have weeds all over the lawn, you can spot-treat or use other environmentally friendly methods of killing weeds.

Final Thought

If you have a weed problem and also need to feed your lawn, you should kill the weeds first, then wait for 10- 14 days before applying fertilizer. Pre-emergent herbicides should be applied before weed seeds germinate, while post-emergent herbicides should be applied when weeds are small and actively growing.

Fertilizing can be done at most five times a year, but you can mulch or use compost to reduce the need for fertilizer.

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