Is Polyurethane Safe for Raised Garden Beds?

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Willie writes on small engine repair, landscaping, and lawn care. He owned a professional lawn care business in Alabama for almost 10 years.

When Is Polyurethane Unsafe for Raised Garden Beds?

Polyurethane is unsafe for raised garden beds before curing. It’s worth noting that polyurethane foam is toxic to plants, especially seedlings. Seedlings exposed to this foam are at risk of tissue injuries.

Before using polyurethane as a sealer for your raised garden beds, ensure it’s fully cured to eliminate its risks to your crops. You can ensure this by doing the following:

Don’t cover the raised garden beds with soil before the polyurethane cures fully.
Keep the raised garden beds in a shady and cool place to hasten the curing process.
Apply thin coats of polyurethane and allow each coat to cure before applying the next one.

Other Ways To Protect Your Raised Garden Beds

Your raised garden beds will rot and wear if not protected from the elements. You can extend their lifespan by using the right methods, as discussed below:

Seal the Raised Garden Beds

Moisture intrusion is one of the leading causes of decay in wood-based raised garden beds (source). It’s common in unsealed wood garden beds.

Applying a wood sealant is an excellent way to extend the lifespan of your wood-based garden beds by protecting them from moisture intrusion.

You can use a water-based sealer or a natural oil-based sealant. Water-based sealers are easier to apply and dry quickly.

A highly adhesive water-based wood sealant like the LR Liquid Rubber (link to Amazon) sticks faster on the wood, shortening your waiting time.

A key thing to remember when applying the sealant to the interior parts of your raised garden beds is to allow proper ventilation. Doing so is necessary to prevent moisture buildup, which can lead to rot and mold growth.

To do this, leave the raised garden beds unsealed at the top. You can also drill small holes on the sides of the bed for better airflow.

Apply a Coat of Paint to Your Garden Beds

Paint makes your raised garden beds look good and also protects them from the elements.

It’s worth noting that not all paints are effective at protecting garden beds from the elements. Choose a high-quality exterior paint that can withstand harsh weather conditions.

The AFM Safecoat Semigloss (link to Amazon) is excellent for painting and priming your raised gardens, especially if you live in an area with a lot of sunlight. It’s a low VOC paint that’ll effectively protect your garden beds from ultraviolet (UV) rays and moisture.

However, avoid penetrating oil or water-based stain and sealer on the paint job as they provide little to no protection against the elements. Also, note that paint won’t stick well on a garden bed that hasn’t been primed.

Other vital considerations when painting your garden beds include:

Sand your garden beds to create a smooth surface for the paint to adhere to. Also, apply a coat of primer before painting to extend the lifespan of the paint job.
Apply thin coats of paint and allow each coat to dry before applying the next one. Doing this will ensure even coverage and prevent dripping or running.

Use Pressure-Treated Wood

Pressure-treated woods contain chemicals that make them resistant to rot, insects, and mold. These woods are suitable for raised garden beds, especially if you don’t want to apply sealants or paint.

When using pressure-treated wood for raised garden beds, always choose those labeled as “safe for gardening use.” These have low levels of chemicals and are safe to use around crops.

You should also avoid using older pressure-treated woods as they may have high levels of chemicals that could leach into the soil and contaminate your crops. Suppose you must use them; then sand down the wood to remove any loose chemicals before using them for your garden beds.

As a general rule, always wear gloves and a mask when handling pressure-treated wood to avoid inhaling the chemicals.

Use Long-Lasting Wood

Building your raised garden beds with long-lasting wood is a natural way to increase their lifespan. Density is an essential factor in determining the lifespan of wood, which explains why wood from denser trees lasts longer (source).

Redwood and cedar trees produce some long-lasting woods excellent for garden beds. Both of these woods have a natural resistance to rot and insects. They can also withstand harsh weather conditions longer than softwood trees.

The only downside to using these woods is that they’re more expensive than other types of wood. But if you’re looking for a long-lasting material for your raised garden beds, they’re worth the investment.

See our recommendations on the 7 Best Wood Options For Raised Gardens (And 3 To Avoid).

Build Double-Walled Garden Beds

Another way to extend the lifespan of your raised garden beds is to build them with double walls. This type of garden bed is also known as a sandwich garden bed.

The double walls provide additional protection against the elements and help insulate the soil, making it ideal for growing crops. They also increase the overall stability of the garden beds.

Frequently Asked Questions

Is Polyurethane Toxic to Plants?

Polyurethane is toxic to plants if used before curing. This finish can kill seedlings by weakening their tissues and causing injuries. Polyurethane is only safe when used after curing.

Should You Seal Raised Garden Beds?

You should seal raised garden beds as doing this extends their lifespans by shielding them from elements like moisture and UV ray. Unsealed garden beds are bound to rot within six months. As a result, it’s advisable to use a seal for undisturbed garden bed usage.

Final Thoughts

Polyurethane is safe for use as a sealant in raised garden beds when cured well. This finish creates a protective barrier against the elements, increasing your garden beds’ lifespan. However, you should give it enough curing time to prevent it from injuring your plants.

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