Why Does Grass Grow Through New Asphalt? 4 Proven Methods to Kill Weeds in Asphalt Cracks

Grass grows through new asphalt due to poor soil preparation prior to laying asphalt. In fact, new asphalt can be ruined within only a few weeks if the ground is not prepared properly. The following factors contribute to grass and weeds pushing through asphalt:

  • Contractors scrape the ground prior to laying asphalt. This removes existing grass and weeds along with the topsoil. Although this is a necessary step, this process won’t get rid of weeds and grass for good. It actually brings dormant, buried seeds close to the surface. These have the potential to grow through new asphalt if proper steps aren’t taken.
  • The soil was not treated properly with a pre-emergent weed killer prior to laying asphalt. If a powerful weed killer designed to kill seeds isn’t used, weeds and grass will push through asphalt.
  • No base or insufficient base was laid before the asphalt was poured. A 4–8 inch gravel base should be laid over properly prepared ground and allowed to settle before it is topped with 2–4 inches of asphalt.
  • Hot asphalt applied over an insufficient base warms the soil below, triggering the growth of dormant seeds. When this happens, grass can begin sprouting through the asphalt within days.

These are important factors to keep in mind when you are having asphalt poured. No one wants an invasion of weeds in asphalt cracks. Make sure any asphalt contractors you consider explain their methods. Ensure they will apply a pre-emergent weed killer and lay a deep gravel base. If these steps are not part of their plan, consider another contractor.

Why does grass grow through new asphalt

How Do You Kill Grass in Asphalt?

Can grass grow through asphalt? Yes, but there are several ways to kill grass growing through asphalt. Below are the most effective methods:

  • Use a long-term weed-killer spray, such as RoundUp 365. Most weed killers wash quickly out of the soil, to allow grass or garden plants to grow. This also allows weeds to return. Choose a long-acting weed killer to keep your asphalt areas weed-free.
  • Try natural weed killer spray. Mix a bottle of vinegar with a quarter-teaspoon of dish soap. This homemade spray will kill weeds and grass growing through asphalt. Keep in mind, vinegar won’t kill weeds to the root, so this isn’t a good long-term solution.
  • Pour boiling water on weeds growing through asphalt. This method is simple and totally natural. A few applications of boiling water will destroy weeds and grass. Similar to vinegar-based solutions, boiling water also won’t kill weeds down to the root.
  • Use a flame weeder. These propane-fueled weed killers are great for use in asphalt areas since there’s little chance of setting any nearby plants or grass on fire.
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Each of these methods is faster and more effective than hand-weeding, which can often be especially difficult in asphalt areas, where grass and weed roots extend beneath paved areas.

Can You Asphalt Over Grass?

You should never apply asphalt directly over grass. Doing so ensures your asphalt surface will be destroyed in record time. Soil should always be scraped and excavated to allow for the installation of a proper base. During this step, any grass and weeds growing in the area must be removed or they will grow through the asphalt.

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Beware of any contractor who plans to pour asphalt over grass, as this will result in a quickly-destroyed asphalt surface.

A single dandelion weed growing through an asphalt driveway

How Do You Stop Grass from Coming Back Through Asphalt?

The key to stopping grass growing through asphalt lies in the preparation. Here are the steps to correct soil preparation and asphalting:

  1. Excavate the soil to remove any existing grass and weeds. Dig deep enough to allow for the installation of base and asphalt.
  2. Roll and compact the soil to ensure a level surface. Surfaces with dips and bumps will result in sagging and cracking asphalt. The grass will push up through these cracks.
  3. Spray a powerful pre-emergent weed killer. This is a special type of weed killer that destroys seeds as they sprout. Regular weed killers (such as RoundUp) designed to kill mature weeds are useless in this application since they won’t kill the weed seeds in the soil.
  4. Lay 4–8 inches of gravel on the leveled surface in stages, rolling and leveling between stages to ensure the gravel is compacted.
  5. If a roller or similar tool isn’t used to compact gravel, allow 1–2 weeks for gravel to settle on its own. Asphalt poured over unsettled gravel will produce low spots and cracks as the gravel settles. This makes it very easy for grass to begin growing through asphalt.
  6. Ensure the gravel base extends beyond the proposed asphalt top by 6 inches in every direction. This will help prevent invasion by weeds and grasses that send runners underneath asphalt, such as Bermuda Grass.
  7. Pour 2-4 inches of asphalt over the gravel base, being careful to leave the 6-inch gravel buffer zone on all sides.
  8. Wait at least 24 hours before driving on new asphalt. During warm weather, you may need to wait longer.

Only by following correct asphalt installation steps can you truly prevent grass from growing through asphalt. By the time grass and weeds are growing through asphalt, the surface has been compromised. In cold regions, water that enters asphalt cracks can freeze in winter, causing buckling and further asphalt damage.

Grass and Weeds Pushing Through Asphalt

When weeds grow through new asphalt, it is almost always a sign of poor installation. When the soil is not prepared correctly and a sufficient base is not installed, you will see weeds and grass growing through asphalt within a few months, weeks, or sometimes only a few days.

Combat any weeds and grass growing through asphalt by using a long-term weed killer, natural weed spray, boiling water, or a flame weeder. This can help keep asphalt damage to a minimum and retain the look of your asphalt surfaces.

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