How To Seal Cinder Blocks for a Garden

When it comes to using cinder blocks in a garden, people have long been concerned about the potential negative health impacts. Though today it is largely safe to use cinder blocks in a garden, some people still feel uncomfortable with the idea of using unsealed cinder blocks in their garden. So, how do you seal a cinder block for a garden?

To seal cinder blocks for a garden, use a waterproof sealant like polymer paint. The sealant should also be weather-resistant. Additionally, ensure the blocks are dry and have no holes or cracks before sealing all the sides. You may add additional drainage to help your plants grow.

Cinder blocks can be incredibly useful when used correctly in a garden. Still, while no longer a health risk, using cinder blocks in the garden can still be somewhat difficult. Read on to learn more about how you can master sealing cinder blocks for use in your garden.

1. Use a Waterproof Sealant To Prevent Water Drainage

If you’re looking for information on how to seal a cinder block, you’ve probably been told of the importance of the practice. However, though many people know that they should seal cinder blocks, they don’t know exactly why they should. 

The simple answer is that cinder blocks tend to absorb moisture through their pores, causing water to drip out of the side not facing the soil. This dries out the soil, making for an inhospitable environment for your plants.

Of course, no one wants to dry out their soil. Thus, many people take the easy step of sealing their cinder blocks so that water doesn’t easily drain from the bed. Unfortunately, most of them make a serious mistake when sealing cinder blocks by using a substance that isn’t waterproof.

If the purpose of sealing the blocks is to prevent them from dripping water, using a substance that won’t block water will be ineffective. Be sure whatever product you use to seal your cinder blocks is waterproof.

2. Use Polymer Paint To Seal a Cinder Block

One such substance you can use to seal a cinder block is polymer paint. Polymer paint is a type of paint designed to prevent whatever material it is applied to from breaking down when exposed to abnormal conditions. 

Polymer paints simply contain an abnormally high number of particles, making the paint apply thicker to the substance it coats. This high number of particles also makes the paint heat, wind, cold, and water-resistant.

When used to seal a cinder block, the paint’s thick particles essentially block the pores inside the cement from either absorbing or releasing any moisture. Additionally, when considering the fact that these paints are being used in an outdoor garden, they can help improve its aesthetic value. Moreover, polymer paints are safe to use, and will not contaminate your garden’s materials.

3. Make Sure Your Sealant Is Weather-Resistant

An important point to consider when sealing cinder blocks used for gardening is that the blocks must be weather-resistant. Typically, when cinder blocks are used for gardening, they are placed on the ground, making them more likely to face natural extremes such as ice, extreme heat, high humidity, or drought. These elements can wear down any material over time. For this reason, you must use a weather-resistant sealant for cinder blocks in a garden.

Beyond the practical benefit of making the sealant last longer on the block, using a weather-resistant sealant can also help prevent the structural integrity of the block from eroding over time. This will make the blocks last longer, preventing you from needing to invest in additional building materials.

Finally, a weather-resistant sealant will also help protect the soil held within the perimeter of the blocks. The blocks will be able to absorb more heat or cold if necessary, helping protect soil quality and potentially extending the growing season.

4. Ensure No Holes or Cracks Exist in Your Cinder Blocks

If you’ve ever driven on a road in an area that experiences a regular freezing and thawing cycle, you might have noticed the decreased quality of the pavement. In places like New England, frost heaves are often roadway engineers’ worst nightmares. 

These are bumps created in the road when moisture enters into the cement, freezes, then thaws. This changes the shape of the road and leads to a number of cracks in the concrete, making the problem worsen with time.

This issue, unfortunately, is not confined to roads. Any structure made from concrete or asphalt can be destroyed by moisture entering cracks via the surface, expanding the hole, and degrading the structural integrity of the cement. For this reason, it is essential to make sure that your blocks don’t have any cracks or holes before you seal them. If they do, either be sure to use extra sealant on that area or go and purchase a new cinder block.

5. Seal All Sides of the Cinder Block

From our discussion so far, it’s clear that you need to seal your cinder block to prevent moisture from escaping and drying out the soil. However, it might not be clear how exactly these cinder blocks absorb moisture. 

Moisture from the soil likes to spread out evenly, meaning that, over time, moisture can collect on the outside of the cinder block. This will eventually enter the pores, causing water to escape.

While sealing a cinder block from the outside might prevent the block from releasing water on that side, it is better if no water is even able to enter the block in the first place. By sealing the cinder block on all sides, you can prevent water from ever entering, or leaving, your flower bed. This will help to keep your soil moist.

6. Make Sure the Cinder Blocks Are Dry Before Sealing

Imagine trying to apply paint to a wet surface. Often, the sealant will be in liquid form, so when applied to a wet surface, the sealant can either become diluted or completely drain away before sealing the cinder block. If you want to make your sealant effective and have it last as long as possible, be sure to apply the sealant while the cinder block is dry.

Taking this step will also ensure that no moisture is left in the block without the ability to drain. If this were to occur, the water would be trapped inside the block, eroding it from the inside. 

The power of nature is unyielding, and though we can try, we can’t ever stop it. If water is trapped inside the block, it will only be a matter of time before the block gets destroyed. Drying the cinder block before applying sealant prevents this.

7. Add Additional Drainage To Help Your Plants Grow

With some exceptions, most plants aren’t able to survive if the soil they reside in is completely soaked. In this regard, cinder blocks can be great for preventing over watering as they will leak out excess fluid. Left unattended, however, too much water can drain, drying out the soil.

In order to create the best results for your plants, seal your cinder blocks so that moisture doesn’t drain out, but add other ways for fluid to drain in case of overwatering. This provides you with the best of both worlds; soil that is neither waterlogged nor excessively dry.

Final Thoughts

When it comes to using cinder blocks in a garden, it is often best to apply some sort of sealant to prevent moisture from draining out of your garden’s soil. Using a waterproof, weather-resistant sealant on a cinder block is the best way to preserve soil quality and maximize the lifespan of your cinder blocks.

When applying sealant, be sure to cover any large holes or cracks, apply the sealant to all sides of your cinder block, and only apply sealant when the block is dry.

Alexander Picot

Alexander Picot is the principal creator of, a website dedicated to gardening tips. Inspired by his mother’s love of gardening, Alex has a passion for taking care of plants and turning backyards into feel-good places and loves to share his experience with the rest of the world.

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