Liners are the Best Way to Prevent Planters From Rotting

The very thing that makes plants thrive will rot and destroy wood planters. To stop wood planters from rotting, you must stop water and fungi from contacting the wood. There are many ways to stop planters from rotting. Each solution varies in cost and effectiveness.

The best way to prevent deck planters from rotting is with a liner. Lining a wood planter prevents water and fungi from contacting the wood. Preventing rotting by separating that which is good for the plants, water and fungi from the wood. There are several ways to line a deck planter ranging in costs, durability, ease of use and customizability.

For a small planter, every fall, emptying and cleaning out the planter will prevent rot. With the wood drying out over winter and fungi slowed without dirt or water. But with large deck planter boxes, we need a better solution. Something that doesn’t involve emptying, removing, hauling out and refilling again. Large deck planter are wonderful as they provide ample room for many plants or larger root bases. These large planter boxes are our focus here. Preventing large deck wood planters from rotting.

4 Reasons Why a Liner will prevent your planter from Rotting

Liners stop rotting by preventing direct contact with soil and water.

Next to sunlight, two things that are essential to plant health is water and fungi. Waters a given we all know we need to regularly water deck plants. But the health of the soil is equally important for plant health. Fungi is what makes the difference in soil health. But fungi are also what rots wood. The challenge is how you can have a wood planter? With healthy soil and water level.

Liners are excellent for this. Liners separate the soil and water from the wood planter. Keeping all the good stuff in the right place. The living soil is moist, encouraging healthy plants. With the wood dry and free from fungi contact. Peace achieved with a little bit of separation.

Air Circulation and Protection with Formed liner

With a formed liner, a little smaller than the planter or with tapered edges air is allowed to circulate on the backside of the planter. Same as your deck, air ventilation is key to both deck and planter health. Air circulation dries whatever moisture gets on the planter keeping it below the magical 19% moisture level, preventing rot. I explain more the importance of ventilation under your deck in another article if you wish to read clink this linkOpens in a new tab.. It is probably the single most effective way to stop rotting and increasing the life of your deck.

Liners prevent wood treatment from leaching into the soil, allowing you to grow edibles.

One of the joys growing plants is eating the fruit of your labour. Decks provide a natural warmer spot perfect for growing juicy tomatoes. I don’t know what it is, but vine ripen tomatoes from your own garden always taste better, bursting with flavour that no store-bought tomato has.

The biggest drawback to eating from your planter is leaching. What kind of chemicals are in your vegetables grown in treated wood boxes? I have read much debate about if the current treated lumber leaching is a problem. Without drawing a conclusion, liners save you from that concern. Plants safely grown in liners prevent the possibility of chemicals leaching from the wood as the soil and water have no contact with the wood.

Lasting longer than wood treatments

Last but not least, liners last longer. Most appearance grade lumber, the stuff you want to use for your planter, will not be treated to a level for ground contactOpens in a new tab.. This simply will not do for dirt to be piled up against. The low level of treatment will allow the planter to rot much sooner than your deck, from contact with dirt.

Yes, you can increase the level of treatment inside the planter put that brings up additional issues which we will get into later. But in short, no treatment safe for plants will last long enough. Your planter will start to rot from the inside out long before your decking and joist will. Especially if you have composite decking.

Before anyone objects and says, but I have…. and it lasted. Yes, I have a raised bed I built out of non-ground contact treated lumbers over five years ago. And it is showing no signs of rot yet and probably won’t for many more years. But Liners allow your planter to last as long as your deck or longer. We are not trying to build temporary planters. If we wanted to do that, better to use portable planters on your deck not built-in planters. Replacing them whenever they begin to rot without damaging your deck. Liners are for life. Or at least a long time.

Quick Selection for liners to prevent rotting.

I will go into much more details, but here is the quick answer for the following categories of liners to select. These are not specific liners sold even though later I will talk about some, but these are about design and material of liner.

The Most Economical Planter Liner

You can’t beat the price of plastic liners. Costing less than a dollar for a small planter. If cost is the determining factor line your planter with plastic. Small planters lined with heavy-duty garbage bags are even cheaper.

The Most Durable Planter Liner

Without going overboard, metal liners are the most durable. Either copper or galvanized, both will last the life of your deck or longer. If you want the liner to last, go with metal. No other liner will last as long.

The Easiest Planter Liner to Install

Going to preformed plastic liner. Metal liners are a close second, weighing just slightly more. I had to pick one. So, the lightest, requiring the least effort or work, is a preformed plastic liner.

Most Customizable Planter Liner

Liquid liners are the most customizable. If you can dream it up and build it. It can be lined with liquid rubber lined. If you can reach your hand and brush in, it can be protected.

The Best Planter Liner

Balancing all four qualities for a planter liner, I select preformed plastic liner. Durable, affordable, easy to install the only aspect it rates poor in is customizable. Most deck planters, even with the most elaborate deck designs in the end “standard” box sizes, are built. They are standard because, most often, they are the most useable size for both the gardener and the plant. A small price to pay for the win in the other three categories. If you really have a unique planter shape, a rubber membrane will be a good solution but will require more skill in installation.

Now, if you want to walk with me through my selection process and know more. For you can decide on your own, which is the best liner for your planter to prevent rot. Here are the seven liner options I considered and my evaluation of them.

7 Planter Liner Options

There are seven planter liners to compare to see which will be best for your deck planter. Comparing the liner’s durability, ease of installation, costs and customizability.

Durability and Cost of Planter Liner

Type of Liner Durability of Liner Cost of 36” Liner
Black Plastic * Less than $1
Rubber Membrane *** $11-30
Liquid Liner ** $45
Copper Liner ***** $215 +
Galvanized Liner ***** $50-150
Formed Fibre Glass **** $300
Preformed Plastic *** $20-70

Plastic liner Durability and Cost

Compared to any liner, this is the least costly. Plastic is cheap and readily available. Ideally, it would be best if you used 6 mil black poly. Thicker poly is more durable than thin drop sheet plastic.

We are told that it takes a zillion years for plastic to decompose or more accurately a 1000 years, plastic bags around 10 to 20 years.Opens in a new tab. Plastic liners are somewhere around that. It will take longer to decompose then plastic bags but won’t last 1000 years. More likely, the plastic will become torn while digging in the dirt before it decomposes. But till then, it’s a very affordable liner.

For a smaller planter, you can use a heavy-duty garbage bag. Commonly only 3 mils, but every bag is two layers making six. Garbage bags are the cheapest liner money can buy. Or if you have a larger planter buying a 1000 square foot roll of black plastic will cost around $60. That’s one big planter or more likely many planters.

Black plastic is both for aesthetic reasons and root health. The black stick up around the edges of the planter will be less noticeable, surprisingly than clear plastic. The solid colour will also prevent any light sneaking through the cracks getting to the roots which is a good thing

Rubber Membrane liner Durability and Cost

The most common rubber membrane liner are designed for ponds. Pond liners are made from flexible rubber, butyl, or polypropylene working to line any size of planter. Rubber membrane quality varies with manufacturers but in short, it is built to hold water. Keeping moist soil from wood should not be a problem. Not unusual to come with a 20 or more years warranty, close to composite deckingOpens in a new tab..

 Home AdvisorOpens in a new tab. puts the average cost at $0.50 a square foot. Putting a 36”x7”x7” planter at less than $11. The struggle is buying smaller pieces for smaller planters at that price. Often sold as a roll product where bulk orders decrease price. Become friends with a pond builder and see if you can have his liner scraps.

If you don’t know any pond builders, on AmazonOpens in a new tab., you can order a 4’x6’ liner. Which you may be able to line two 3’ planters. If carefully cut and placed.

Another option for rubber membrane is to use the 1’ rolls of joist membrane or roofing snow and ice shield. Strong durable designed to protect shield wood from water. With a sticky side to adhere to the planter. Keeping the liner against the planter not curling up on the edges making a mess. The biggest trouble is adhering the membrane to the wood. It is extremely sticky, and it can be hard to pull off the film and keep in place. And once it sticks to itself, throw it away, it is next to impossible to pull apart without damaging.

Liquid Liner Durability and Cost

The adhesion of joist membrane without the struggle of it sticking to itself. Coming in a can and a non-sticky seam roll for joints. Rated low not because of the quality of the product but the application. The liquid membrane is only as good as it is applied. If applied under the correct condition with great care for details, covering the wood consistently, it will protect the liner for years. If not well, that’s why it’s not rated so high.

More costly than rubber membrane but similar in reducing cost with larger planter. Buying a gallon gets you four times as much at only twice the price. But only of value if you have enough deck planters to use a gallon.

At Liquid Rubber, they have a few different options of sealant, seam tape and kits. If you would like to order, click hereOpens in a new tab. to link to their website. If you prefer Amazon Opens in a new is their direct link.

Copper Liner Durability and Cost

Copper is extremely durable and resistant to corrosion. Even when dropped or bent, it will hold together preventing leaks. The Canadian Parliament Roof is constructed of copper, was installed in 1916 lasting through acid rain, hail and political storms below and except for greening is still protecting.

But this durability comes at a price, being one of the more expensive planter liners. At Window Box, you can order a 36” x 8″W x 7 1/2″ H copper liner for $215. A nice-looking durable planter but pricey. It may be better to not cover put display it on your deck in all its beauty. Click here to order from Window Box.Opens in a new tab.

Galvanized Liner Durability and Cost

Similar in durability as cooper but considerably less money. Galvanized liners price variation is in the finish. Cooper tone liners look like cooper but three quarters the price. A galvanized liner powder coated in white or bronze costing a quarter of the price of a similar size cooper liner. Being competitive in price as plastic liners but with increased durability.

You can order a galvanized liner from Window Box also. Click here to see their selectionsOpens in a new tab..

Custom Formed Fibreglass Liner Durability and Cost

Fibreglass liner is a very durable product but can break from impact. When surrounded by a wooden planter, it will last for years. Protecting the wood from rot and holding your beautiful plants.

The most expensive liner on our list. Cost has more to do with being custom then with the material cost. Fibre Glass is better suited to large custom planters, but I am getting ahead of myself. If you want a “standard” size shallow planter, a plastic liner will be more affordable.

If you don’t have a local fibreglass shop here is a link to Planters UnlimitedOpens in a new tab. for a custom fibreglass manufacturer specializing in custom planter liners.

 Preformed Plastic Liner Durability and Cost

The most readily available planter liner. You can find them at your local gardening centre, building supply store, Walmart or even here on AmazonOpens in a new tab.. Not as rigid and durable as fibreglass but with the same weakness, impact. When sheltered inside a wooden planter, they will last for years. But with thin walls, they can be brittle with impact.

Some better-quality ones will have a stronger shell, allowing you to use them either as liners inside a wood planter or stand-alone.

The range in price comes from features. Stronger walls are more durable but increase the cost. Some also coming with a built-in watering system. Allowing you to water your plants less without them drying out. Saving you time, reducing worry and plant stress from lack of water.

Lechuza Balconera Self-Watering Planter has both a durable shell and self water feature.Opens in a new tab.

Earthbox, also sold at Amazon, is a complete kit to help you grow better vegetables. A 29″ L x 14″ H x 11″ liner with accessories.

Either way, plastic liners are fairly durable for the price. Being one of the lower price liners out there.

Now for what be the more important aspect of plant liners, ease of installation and customizability. What will it take to install the liners regarding time and skill? Combined with, does the planter have to fit the liner, or will the liner fit the planter?

Planter Liner Ease of Installation and Customizability

Type of Liner Ease of Installation Customizability
Black Plastic ** Limited to Sheet Size, Fold
Rubber Membrane *** Limited to Sheet Size, Fold
Liquid Liner ***** Unlimited
Copper Liner * Standard sizes
Galvanized Liner * Standard sizes
Formed Fibre Glass ** Can be ordered to Size
Preformed Plastic * Standard sizes

Let’s get into the detail of each planter liner. How are easy are they for installation and will they fit our imagination for planter shape?

6 mil Black Plastic Installation and Customizability

How well can you fold? Are you the one that loves wrapping presents? Neatly folding the edges, creating the perfect wrap every time. Seeing the fold that makes all the difference in appearance.

Both how easy plastic is to install and what shape of planter you can line depends on the fold. Installing plastic is the same as wrapping presents except it’s the inside, not the outside your wrapping. Folds are best as they limit potential leaking points, with continuous plastic not patched together pieces.

Where cutting and taping together pieces creating a potential leaking point. Some corners will require a straight cut being taped up with poly tuck tape to reseal. But the less cutting the better.

Using a ten-foot-high roll will allow you to cover almost any size of planter box. Laying out the poly over the top the box, cutting it larger than the planter and carefully folding and pushing into the planter, conforming its shape. Then you can either staple the plastic near the top of the planter on the inside. Or simply clip and fill with dirt, allowing the dirt to hold the plastic in place. Trimming to size once the planter is filled within a couple of inches of the top.

One of the advantages of plastic liner while installing is cost. If it tears while installing simply replace it with a new sheet. Only pennies not hundreds to replace. Besides, smaller pieces can be used for smaller planters.

Rubber Membrane Installation and Customizability

Simpler to installing plastic liner except that the membrane is a little more rigid. I say a little as it is still quite flexible and easy to manipulate to the planter shape. Again, lay over the planter and carefully work your way around pressing and folding the membrane into the planter. Clip or staple along the top once in place, or again let the soil hold it. An alternative is you can use 3” joist membrane to tapeOpens in a new tab. it along the top. Providing a good seal while cleaning up the edges.

Being more expensive then plastic, rubber membrane will need to be carefully selected for size. Select a size that covers the bottom and sides fully. Which is limiting in customizability. With an odd shape, like an “L” shaped planter, you may need to use a very large piece to fit the shape. Conforming a rectangle to a bend.

Or you can use self-adhesive double-sided tape to sealing overlapping pieces. Reducing waste while keeping a complete seal. Carefully dry-fit together all your pieces and overlap. Once everything is in place, fitting and overlapped sufficiently, carefully apply the tape without shifting the membrane pieces. Firmly pressing the pieces together, ensure complete contact and seal.

If you need two sides pond tape, here is a link to some on AmazonOpens in a new tab.. There are some other alternatives to connecting the membrane if you like. The biggest thing is taking care to overlap correctly and fully conform to the inside of the planter. Standard shapes are easier but with a little imagination and sealing, you can line any deck planter.

Liquid Liner Installation and Customizability

Patience is a virtue, and patience will greatly help with using a liquid liner to seal a wood planter. Grab a brush, some gloves and begin to paint. And paint you will, rubber membrane can take four to five coats to build sufficient mass. Protecting the planter from water and fungi, preventing rot.

This is the only liner included on this list that is not a put into the planter but painted on. I included it as a liner, not a paint or stain because you are actually building the liner in the planter. Building the membrane in place, on the planter floor and walls.

The skills and patience required with multiple coats are its challenges in installation. Not trying to fit a square peg into a round hole as with rubber membrane but patiently applying multiply layers of rubber. Building the membrane inside the planter. But the membrane is only as protective as the care is taken in the application. Covering and protecting the entire inside of the planter.

Liquid liner’s challenge in applications is its greatest strength in customizability. If you can paint it, you can line it. Allowing almost any planter shape can be lined. Nothing is off-limits. Everything can be lined and protected with liquid membrane and seam tape for the joints. Or another way to look at it. Liquid membrane is a rubber membrane custom in design. Customized to your creative deck planter.

Copper and Galvanized (Metal Liners) Installation and Customizability

We are going to discuss copper and galvanized liners together because, besides their price difference, which is significant, they are the same. If you can double up your coffee cup, you can install a metal liner. Preformed, sturdy and easily sliding into place. Metal liners cannot be easier in installation.

But this ease in its installation is its weakness in customization. In short, there is none. Unless you are going to pay considerably more for a custom liner to be fabricated, the size you order is the size you get. If it’s 36”, it will not fit your 34” corner unit. There is nothing you can do to change that. What you see is what you get.

Which, with a little foresight is not a problem. Before building your deck, while still in the design stage. Designing the built-in planters according to the available sizes, can easily be done. Meaning that you need to select and order the liners before building your deck and planters. Then during construction, confirming sizes. So that everything is built to fit the liners. Customizable? No, but can be worked around. If you preselect and design your planters.

Fibre Glass Liners Installation and Customizability

The actual installation cannot be any easier. Similar to metal and plastic containers, being dropped into place. The additional challenge is measuring. The opposite of metal liners. Where the metal liners you build the wood planter for the liner. With custom fibreglass liners, the liner is built for the wood planter. After the fact. Measuring the inside of the planter accurately. Communicating these numbers to the fabricator correctly in that the correct size and shape to be built.

A 1/8” incorrectly measured length can be a disaster. With the new custom liner not fitting. This can be accounted for by allowing for space around the liner. But the fact still remains that your measurements and notes must be clear and accurate. For what you want is what you order and what you get.

Plastic Containers Liners Installation and Customizability

Preformed liners and again, your planter must be built for the liner not the liner for the planter. Making them super easy to install. Sliding one box into another box. The planter box can be designed to dress up the liners a bit, but it remains that it’s a preformed box slid into another.

Similar to metal liners except in the price and features. Being one-tenth of the price of copper and over half the cost of a galvanized liner. Going plastic saves money.

Many plastic liners are designed with self-watering systems which are great for plant care. But these features come at a cost. The cost is customizability. There is none. All deck planters with plastic liners must be built for the liner. There are many preformed options available. Giving many options for planter’s shape and sizes but none the less. It must conform to the liner.

Larger Plastic Planter options

Many plastic planters are designed for small rail planter. Meaning they are limited in size and usability for a larger deck planter. Like that Ornamental Crabapple in the centre of the deck. The centrepiece of the deck, bringing life and beauty to the entire deck but will not fit in a 36” x 7” planter.

For those situations, you will need to order a larger planter suitable for the plant and planter. Some garden centres will stock larger liners. You can also order them from Deep Stream which as many larger container sizes in stock. Here is the link for Deep StreamOpens in a new tab..

Drainage of Planters

With all this talk about the protection of planters. Installing liners to prevent water from rotting the planters a little reminder. Planters need drainage. Plants do not do well sitting in a puddle of water. Healthy soil needs to drain, allowing the water to moisten the soil but not drown the plants. To achieve this all liners, need to be able to drain. Excess water needs to drain away not pool, drowning and rotting the roots.

Drainage can be achieved in one of two ways. Small holes on the bottom of the liners and planter, allowing water to drain out. Care must be taken to redivert this water away from the planter and deck. You don’t want the water running out of the liner and rotting the planter and deck. Small pipes collecting and directing the water away from the decking is often the answer.

Another option is a built-in reservoir for the planter. Providing a place for the water to pool, away from the roots. Plastic self water planter are designed for this. With small bottom reservoirs for both excess water to pool and also watering the dry soil. Protecting the plants from both extremes to much water and not enough. A healthy balance for plants and decks.

You can also create a reservoir in the bottom of the liner. Most plants only need roughly 8” of soils for healthy root development. The bottom of the liner can be filled with drain gravel, plastic bottles or a false bottom. Anything that will provide room for water to pool away for the roots. Cover with landscape fabricOpens in a new tab. to prevent soil from filling up the gaps, then cover with soil. Creating a place for excess water to run but keeping the roots out.

Why not stain to protect the wood?

When first thinking about how to prevent deck planters from rotting, my first thought went to treating the wood. Thinking and researching organic sealants and products that are safe for plants and prevent the wood from rotting. But the more I thought, the more I researched the more I came to realize. Liners are a better solution.

Liners last longer than almost any wood sealant or stain. A lot of the ones that last longer are not good for plants. Or are not designed for ground contact, destroying their value in planters. The best and most versatile solution is a liquid liner. Being applied similar to stains and sealants but actually building a membrane of protection. Lasting longer inside the planter while in contact with soil.

Enjoy your plants and deck

Wow, that was a lot! In the end, I want to say this. Enjoy your plants and deck. Experiment with what works best for you. Balance out your wants and needs of your deck. But above all, decks are to be enjoyed and your plants with them. Have Fun!

Ryan Nickel

A Red Seal carpenter, passionate about building decks to be enjoyed.

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