Planter Drainage, Essential for Plant and Deck Health


Plants bring life, increasing deck enjoyment during the summer. Either built-in planters, planter pots, or hanging all are great on your deck. Plants need water to live, but to much water will kill them. Planters must be designed with this in mind. Designed to supply thirsty plants with water but removing excess water to prevent drowning.

Introduce deck planter box drainage, the answer to the challenge of water extremes. Draining excess water from the planter, but more importantly, exchanging of oxygen and water within the planter soil. Drainage of excess water is logical, but more importantly, the planter soil needs to breathe for the plant can breathe.

Unfortunately, pooling water on a deck from planters will damage the deck. Deck planters must have drainage for the plants to thrive, but draining water on the deck can damage the deck. Which raises this important question.

What to do with excess water in deck planters?

Water is one of the greatest threats to a deck. Excess water causing rot, damaging your deck and shorten the life span of your outdoor living space. Shortening the life of the source of so much enjoyment is not good. Planter drainage is essential, but dealing with the water is essential for both plant and deck life.

For the solution, we must look to nature. Plants are natural. Water is natural. Soil is natural. Wood is natural. So how do we get them all to work together? We look to nature and how God arranges everything to work together. The cycle of water if I may call it.

Water is both the solution and the problem. In nature, excess water is diverted or collected to be evaporated. To protect the deck and have our plants thrive, we must do the same.

Drainage Starting with the Planter Soil

When I first heard about planter drainage, my mind went to holes and drains. Getting excess water out of the planter, away from the plant roots. But the more I researched and thought about it. We need to take a step closer. Before we get to draining planters, we need to talk dirt.

The soil around the plant has more of an impact on planter drainage then holes in the bottom. Soil that holds water but still allows water to drain, creating air pockets. Remember, in nature. It’s the fertile soil that makes the difference in plant life. Yes, drain holes are necessary for large amounts of excess water, but the micro-level of soil moisture around the roots is critical.

First, I want to say I am not a soil expert. I build decks, but I love plants and blending the two. What I have read and makes sense is that planter soil needs to drain. Allowing excess water to drain away from the plant’s roots while still holding enough water through the heat of the day.

Eric Vinje, owner and founder of Planet NaturalOpens in a new tab., provides this classic planter soil mix.

  • 1 part peat moss or mature compost
  • 1 part garden loam or topsoil
  • 1 part clean builder’s sand or perlite

In short, something to hold water, the peat moss or compost. Something to drain water, creating air pockets, the sand. Plant roots must receive oxygen. And fill for the roots and planters, loam. A horticulturalist may correct me, but that’s how I understand it. Do this, and your plants will live. For small planters or simple convenience, you will often be better off buying organic potting mix. Where they have done all the science and mixing for you.

Having good soil for our deck planters, we can now turn to planter and drainage.

Draining the Swamp, allowing water to drain from your planter

We must get excess water out of the planter. Even with good soil, excess water needs to be removed from the bottom of a planter. Allowing water to flow through and preventing accidental overwatering. Saving the plants from drowning. If your planters don’t come with drain holes. Holes will need to be drilled.

Drain holes also remove waste from the soil. Washing away the plant’s waste. This is one of the main reasons you don’t want to trap water at the bottom of the planter. Not only can you drown the plant put trapped water collects waste. Ever notice how trapped water grows slim and a bad odour. Water needs to be moving along the water cycle. Watering plants and evaporating through the leaves or moving to its next place of need. Not trapped in planters, collecting bacteria and smelling.

The one exception is self-watering planters, where new water is added not from the top but pulled from the bottom reservoir. Providing a constant source of water from the reservoir through capillary without drowning the plants with excess. Again, good planter soil is essential.

Holes at the bottom of the planter is critical

Holes on the bottom of the planter will drain excess water after the soil had become saturated or dumps of water. Preventing a swamp of dirty, excess water from sitting in the bottom of the planter. Drowning the roots. Also, since you can not see the bottom, you may water a planter that’s drowning the roots below. Because the top of the soil is dry. By draining excess water, you know that there is no water sitting on the bottom.

Planter drainage holes should be between ¼” (6mm) and ¾” (18mm). Big enough to allow water to drain but not the dirt.  A good quality soil will help here. Quality potting soil has enough cohesion to allow water to run through will holding itself together as not to run through the drain holes.

To help prevent soil from running out of the drain holes, especially large ones. Covering the drain holes with plastic mesh or weed barrier will help to hold the soil in. The key with built-in planters is durability. You don’t want it to break down a few years down the road after filling the planter full of dirt.

For plastic mesh, using a scrap of window screen is both durable and economical. The plastic will not decompose, keeping the soil in for years. But the holes are large enough to allow water to drain.

Landscape Fabric is my favourite. It is designed for water to freely flow through while lasting for years buried under the dirt. Plus, already having landscape fabricOpens in a new tab. from using it to stop weeds under the deck. Which I highly recommend in this articleOpens in a new tab. about under deck treatment. A small piece at the bottom of the planter prevents any soil from running out while allowing excess water to drain.

Location of Built-in Planters Drain Holes to Prevent Damaging the Deck.

Planter drain holes should be drilled every 6″-9″ (150-230mm). Close enough to allow sufficient drainage. Any more will not increase drainage but only weaken the planter floor.

Decking is gapped to allow for water drainage, which is excellent for excess planter water. The deck already has a built-in drain. With built-in deck planters, line up the planter drain holes with the decking gapping, allowing water to drain out of the planter and unto the ground. Returning to the beautiful water cycle, all without damaging the decking.

If using liners with larger drain holes to one side, you can drill a similar hole in the decking to allow water to drain without pooling on the decking. Pooling water will damage the decking.

Speaking of liners, liners are essential to planter and deck health. Providing a layer of protection preventing rot. I have written an article discussing seven different liner options, comparing durability, ease of installation, costs and customizability. If you would like to learn more, click hereOpens in a new tab..

Dealing with extra water under your deck

Regular watering of planters greatly increases the amount of water under your deck. It’s like running a sprinkler under your deck every day. Great to water the lawn daily but bad for under your deck. Water increase wood rot, rot destroys wood decks. I explain more about preventing joist rotting hereOpens in a new tab., but dealing with water is key.

With a higher deck with plenty of ventilation, extra water will not be a problem. Ventilation is the best way to prevent wood rot. The sun and air circulation will deal with the water. As long as the water is directed between or off the decking and joist.

For lower decks with poorer ventilation helping to move the water is helpful. I have seen many methods to remove excess planter water from under the deck, from attaching hose or pipes to the bottom of planters to carry the water away. The simplest is attaching an eavestrough under the drain holes below the deck. Catching and carrying the dripping water away from the deck, keeping the ground under the deck dry.

Drain holes, Eaves troughs, ventilation under built-in planter are all great for large planters, but what about small planters? With small planters, we need to use a few different tricks.

Small Planters Drainage

With smaller moveable planters, we need to solve the problem of drainage a little differently. As the drain holes may not line up with the decking gaps. Or water collection system under the deck. For smaller planters to not damage our decks, we need to utilize collection and ventilation.

Collection of excess water draining from a planter

Preventing running from pouring at of the planter and pooling on the deck water collection is helpful. The two best ways are double potting or saucers. Both need to be removable and allow you to see the water level.

It is very important to be able to see how full the reservoir is, seeing when you need to empty it. Or allowing you to water your plants without the added step. In the words of G.I.Joe, “knowing is half the battle.” The other half is emptying the reservoir when needed, and that is where removability is so important.

Removeable allows you to safely empty excess water, ensuring that it doesn’t spill unto the deck. With a connected saucer, you may see that it’s full, needing to be emptied but without any way to drain the water without damaging the plant. Draining an attached saucer required you to tip both the planter and plant. Something is going to get broken or spilled. A removable collection allows you to safely pick up the container holding the plant and soil, set aside. Empty the water reservoir and return. All without damaging the plant or spilling water and dirt all over your deck.

Saucers for deck Planters

Not some new wonderful invention. You probably saw your grandmother do this with her houseplants and her with her grandmother. Saucer provides a nice collection point under a planter. One that you can see the water depth without moving the planter with the added feature of room for evaporation. Saucers slightly larger than the pot will collect water, which can evaporate out of the pan, saving you from emptying them. Using a clear saucer makes it even easier to see the water depth.

Double Potting is Water Collection with Style

Similar to saucer but more for appearance. Or as I heard another author put it, a decorative cachepot. Something tells me that’s probably an old name, but that is essentially what double potting does. It’s a decorative way to catch water draining from your planter.

We have all seen them, those cute kitten pots, or elves or anything that is eye-catching but simply is not good for actually planting your plants but looks great on a deck. These are ideal for double potting. Something that simply pops but isn’t practical to fill with soil. Often impractical to drill drain holes in, but still are wonderful double potting planter. Plant them in a plastic liner with proper drain holes, set them in the decorative pot to collect excess water and add beauty to your deck.

Ventilation under deck planters

Water is fine on a deck. Decking is designed to get rain on it. The key is, can it dry out. If it can safely dry out regularly, rot and mildew will not be an issue. Planter sitting on a deck needs to be regularly moved or raise above the decking to allow the decking to dry.

Ventilation must accompany drainage, ensuring that water draining out of the planter doesn’t become trapped under the planter. Trapping water, even a small amount, will damage the decking. Raising the planter, allowing drying air underneath, prevents rot and mildew from happening.

In Risk and Pleasures of Planters on Decking, I discuss more of the dangers and solutions for planters on decking. If you wish to learn more, click hereOpens in a new tab. to read the article.

Double potting should be selected on adding distinct planters reflecting personal taste. But if you just want to order a raised saucer for your planters, here are two links.

First is a clear heavy-duty saucer with legs. Remember the advantages of seeing the water level.

Having wheels under the saucer is advantageous in moving the planter around the deck. Better for drying out the deck and cleaning under the planter. Also, on Amazon is this planter caddy with locking wheels.

Gravel in Non-Draining Pots

There are so many beliefs and discussions about gravel in planters’ pots for drainage. It didn’t seem right not to have at least a short discussion about it.

First, if it is working for you, have at it. I am not here to tell you how to garden on your deck, but currently, most experts are of the consensus that gravel at the bottom of planters does not help with drainage.

The strongest argument being that water follows fine particles and does not make the leap from potting soil to gravel until complete saturation of the soil above the gravel. In short, it is not providing drainage but only reducing the soil level of the planter. Whatever you fill with gravel is that much less actual draining soil you are removing from the planter.

Secondly, if by some chance the water does drain from the soil into the gravel. Now what? What happens to the trapped water at the bottom of the planter? It cannot go back up into the soil. Sorry, water is powerful, but gravity is stronger. The water sitting, trapped in the gravel below the soil without drain holes to escape. Remember trapped water, is bad. A gathering pool of waste and bacteria at the bottom of your planter.

If the first two are not true, we still have a problem. You also can’t see how much water is at the bottom of the planter in the gravel. Giving you no idea if you need to water your plants or not. If the gravel is full of water, watering will drown the plants. Or assuming they have water, and they are drying out without seeing.

In short, besides adding weight to the planter, gravel is not helpful. If the wind is knocking over your planter, gravel may be an excellent solution, but regarding drainage, stick with drain holes.

Summary of Deck Planter Drainage

Water is living and must stay moving, either through the soil or up the plant stems and leaves. Planter drainage is essential, but the water must be diverted away from damaging your deck or collected. Air and water are a beautiful team both in plant health and countering excess. Design your planters that both can work together. And above all, enjoy your deck!

Ryan Nickel

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

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