Is Polyurethane Ever Safe for Gardens?

Polyurethane is a helpful type of plastic that has many functions. It’s often used as a sealer for wood furniture and other objects, but it contains plenty of chemicals. So, is it safe to use in the garden, or should you use something else instead?

Polyurethane can be safe for gardens if kept away from people and animals while it’s still curing. Once fully cured, polyurethane isn’t toxic for the most part and is safe. However, it may remain slightly toxic to animals, so you should try to keep pets away from polyurethane as much as possible.

The rest of this article will discuss the safety of polyurethane in the garden in greater detail. It will also discuss how to know when it’s fully cured and other safer alternatives.

When Is Polyurethane Safe for Gardens?

Polyurethane is safe for gardens when it’s fully dry and cured. Before it cures, it releases toxins and chemicals into the air that aren’t safe to breathe in or consume. If using polyurethane for garden beds, avoid planting anything inside until it’s fully dry.

The most common use of polyurethane in the garden is sealing paint on wooden furniture and gardening items, like garden beds. Since you’ll be placing plants or food seeds into these garden beds (and possibly consuming the product at a later date), you want to avoid having any wet polyurethane near your crops.

Once you give the product adequate time to dry and cure, you shouldn’t experience any issues.

Can You Apply Polyurethane to the Inside of a Garden Bed?

You can apply polyurethane to the inside of a garden bed, but it’s not the best idea because it can be toxic. Instead of using polyurethane, you could use a less toxic alternative like natural oil. However, if you decide to use polyurethane, make sure it’s water-based.

You can also purchase polyurethane in oil form, though this one has a longer drying time. When sealing the inside of a garden bed, you must give it plenty of time to dry before planting any soil or plants inside. This ensures the chemicals won’t seep into your soil or crops and keeps everything safe and healthy.

If you’re worried about toxicity, you could avoid sealing your garden bed altogether. However, it’s good to be aware of the benefits of sealing it. They include:

  • Reduced risk of mold growth.
  • Rot prevention.
  • It helps the garden bed last longer.

So by not using a sealant (like polyurethane) to seal your garden bed, it likely won’t last as long and might begin to rot sooner. Since plants and soil can often be moist, rotting and mold growth are likely. The sealer will prevent this from occurring.

When you haven’t sealed wood, it’s more likely to break down and get damaged. Some bugs might be attracted to unfinished wood, especially if holes or gaps form over time.

But this isn’t the only thing that can attract bugs to a garden. If you want to learn what else attracts bugs to gardens (as well as how to prevent them), you can check out my other article for more information: Do Rock Gardens Attract Bugs?

So it’s easy to see that using a safe sealer for wood in your garden will be highly beneficial!

Is Polyurethane Toxic To Plants?

Polyurethane can be toxic to plants in some cases, including when it is still wet. Other more solid forms of polyurethane can also be harmful to plants, so it’s better to use alternative products.

A study by the National Library of Medicine states that although polyurethane-based products like foam plugs are helpful in the garden, they can be toxic to plants. This is particularly true for tiny seedlings.

Therefore, from time to time, plants that have constant contact with polyurethane products can experience adverse effects, such as death and rot.

Another thing to consider is phthalates. These are chemicals commonly found in plastic products (including polyurethane) and known to be toxic to humans and the environment. Although not all polyurethane contains these harmful chemicals, you can never be too sure.

Since phthalates are harmful to humans, you can imagine they’re damaging to plants, too. Because of this, it’s best only to use sealers and other gardening products labeled as safe for plants.

Safer Alternatives to Polyurethane Sealer

Although polyurethane-based sealer products aren’t always harmful to plants, you might still be looking for a safer alternative. Luckily, there are many different options for wood sealers, and many of them are safe and less toxic than plastic products.

Let’s look at some of them below:


Although some people may think of manicures and pedicures when they hear the word shellac, one of its most popular uses is as a wood sealer. It’s easy to apply and non-toxic, thus makes an excellent alternative to polyurethane. Plus, you can usually choose between different colors. 

Since most wood is a shade of brown or tan, many shellac sealers come in brown shades. You can also opt for a clear shade that will suit any wood color.

Because it’s non-toxic, shellac is also acceptable to use on garden beds and other items around the garden.


Lacquer is another alternative to polyurethane. However, it does contain some toxic chemicals if consumed or inhaled in large quantities. But if you’re applying lacquer to furniture in your garden, you shouldn’t have to worry about these fumes because you’ll be out in the open with fresh air.

Additionally, lacquer dries faster than polyurethane.

Flaxseed Oil

Flaxseed oil is a highly popular and excellent alternative to polyurethane sealer (also known as linseed oil). This oil is popular because it protects wood from damage plus wear and tear while remaining completely non-toxic.

As a result, it’s perfectly safe to use around your garden. It’s even safe for animals, so you don’t have to worry about anyone getting sick.

Walnut Oil

Another natural product that’s a safe alternative to polyurethane is walnut oil. This is yet another sealer that is non-toxic, and you can even use it to seal wooden objects used for food (such as wooden bowls and plates).

You can use walnut oil to seal many things in your garden, including wooden furniture and garden beds. It dries quickly, particularly in hot, sunny weather.

Can You Apply Polyurethane Over Paint?

You can apply polyurethane over paint because it acts as a sealer. However, you should make sure the paint is fully dry before applying polyurethane on top. If it’s not dry, you’ll ruin the paint job, and the products will start to mix.

Additionally, if the paint is wet, the polyurethane won’t be able to do its job as intended because the wet paint will get in the way. You certainly don’t want the wood in your garden to look imperfect and unfinished, so always make sure the paint is dry before sealing!

Applying Paint Over Polyurethane

Sometimes, you might need to apply paint over wood that’s already sealed with polyurethane. This could be a wooden chair, flower bed, or basket. Whatever it is, you can paint over the polyurethane sealer as long as you prepare it first. It’s entirely safe to do so, and you shouldn’t face any issues once you do it right.

To prepare the surface for paint, you’ll need to sand it to have a more rough texture. Sanding will remove the smoothness of the finish, making it easier to apply paint. After sanding, you’ll need to clean the surface and make sure there are no leftover pieces of dust or dirt. Then, you can paint it. 

Once you’ve painted the wood and allowed it to dry, you can apply some polyurethane (or another sealer) on top.

How To Know When Polyurethane Is Cured

It’s hard to know when polyurethane is cured by merely looking at it. Many people assume that once it’s dry, it’s also cured. However, this isn’t the case. Although it will usually dry within 24-72 hours, polyurethane can take around a month to cure fully.

Since polyurethane is generally considered safest once it’s fully cured, it’s best to wait around a month before you start using your wooden objects (especially if they’re objects that will be holding soil and plants).

After that time has passed, it should be okay to use your garden furniture (or whatever else you plan on using) without concerns.

However, some people will begin using their sealed items once the polyurethane has dried. You can do this if you wish, but be sure to read the label on your sealant beforehand because it’s essential to be aware of any possible risks.

Polyurethane and Animals

For the most part, polyurethane-based products are not safe for animals. Therefore, if you have pets that roam around your garden, it’s best to keep them away from any polyurethane.

Some polyurethane products can contain isocyanates, which cause cancer in some animals. Of course, this doesn’t mean that your pet will get cancer from coming into contact with polyurethane in your garden, but you should exercise caution.

If you have animals present in the garden most of the time and they’ll be in constant contact with polyurethane, you should opt for a different sealant.


Polyurethane is safe for gardens when it’s completely dry and cured. You shouldn’t place soil and seeds in a flower bed if the polyurethane sealer is still wet because it can be toxic to the soil and plants.

Although it’s generally safe to use a flower bed a few days after applying polyurethane, it’s even safer to wait until it’s fully cured (which takes around 30 days).

There are safer alternatives to polyurethane that you can use. They include flaxseed oil and shellac, and they work just as well as polyurethane.

You can read my other article on the best time to paint your garden fence here: When’s the Best Time to Paint Your Garden Fence?

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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