Drainage in Planter Boxes? (Number, Size)
Did the planter box you recently bought come without drainage holes? Are you suddenly wondering if you should drill a couple of holes in there you should leave it as it is?
Depending on the size of the garden planter, one to four 0.25–1-inch (0.63–2.54 cm) drainage holes are recommended. These can easily be done by drilling through the underside of the planter box, especially if it is made of plastic, wood, or metal. Drainage holes are essential to prevent root rot.
Just to be clear…
Before moving forward a small consideration is needed: gardening and especially the number of holes in a planter is not an exact science. The values below are just recommendations based on our and other expert gardeners’ experience.
So, if the size of the holes or number is not exactly as we do recommend no worries. Doing those holes is the key, as you cannot leave the planter without them, especially if outdoors.
Are Drainage Holes Necessary for Planter Boxes?
Planter boxes must have drainage holes to prevent overwatering. The lack of drainage holes increases the likelihood of the soil being too moist, preventing the roots from absorbing oxygen and causing the plant to drown from root rot.
Planter boxes of all types require drainage holes! At the very least, it is still recommended to drill a drainage hole into the planter if they don’t already have one. The only case in which drainage holes can be avoided is for a gardener that monitors daily his plants in a dry area.
Indeed, for outdoor planter boxes where rain and other environmental factors can easily cause overwatering and subsequent drowning of the plant.
Drainage Hole Size in Planter
Drainage holes should be 0.25–1-inch (0.63–2.54 cm) in diameter. Larger drainage holes encourage better draining due to fewer chances of blockages caused by soil and debris.
Drainage holes can be made by using a drill with the proper drill bit. A large nail and hammer can also do the job. Drainage holes don’t need to be pretty or clean since these won’t be seen. However, a gardener may sand off the rough edges and imperfections if a clean finish is desired.
Note: Putting drainage holes on tougher materials like plastic, fiberglass, and wood is less delicate work since they can handle the force of drilling or hammering. Brittle materials such as ceramics or terra cotta need to be handled much more delicately since they may shatter or break under force.
Drainage Hole Location in Planter
It is recommended that drainage holes be spaced evenly along the bottom length of the planter box. The drainage hole spacing is not all that important since it depends on the planter box’s size and the gardener’s preference.
Drainage hole spacing is only really important when the material of the planter box is brittle or weak such as terra cotta or ceramic. The structural integrity of these materials is compromised when the holes are placed haphazardly.
Example: Avoid placing multiple holes in stress areas like the center of mass of a rectangular planter box made of weaker materials. This is important in the case of a planter with legs and the bottom of it not resting on the ground.
Number of Holes per Planter Box Type
As a general rule, it is recommended to make one to four (1-4) drainage holes in a planter box without compromising its structural integrity – taking into account the material it is made of.
Example: Weaker materials should only have one to two (1-2) large holes whereas durable materials can have as many holes, both large and small, which they can manage.
1 – Terra Cotta
Terra Cotta clay planters should have at least one or two 0.25–1-inch (0.63–2.54 cm) drainage holes at the bottom or near the bottom sides. Terra cotta clay, glazed or unglazed, can be quite fragile so minimizing the number of drainage holes will maintain its structural integrity.
Most terra cotta clay planters available in markets already have drainage holes at the bottom or at the sides. If there are none, it is recommended to drill one with a carbide masonry drill while pouring a continuous stream of water on the drilling area.
Note: Terra Cotta is earthenware typically shaped from clay and heated at high temperatures to hold its shape. They are affordable and accessible in many markets in the world.
Terra Cotta planters come in 1) glazed or 2) unglazed. Glazed terra cotta planters are applied with a special, clear, high gloss sealer which effectively prevents moisture from entering or leaving the terra cotta clay. The unglazed terra cotta planters are more fragile and less water-resistant due to the lack of high gloss sealer application.
2 – Ceramic
Ceramic planter boxes should have one or two 0.5-1 inch (1.27-2.54 cm) drainage holes using an electric hand drill with a carbide masonry drill bit. Ceramics are hard yet brittle so it is best to minimize the number of holes to maintain the structural integrity.
Drainage holes can be made with a drill with the proper carbide drill tip. Start drilling from a 45-degree angle which gradually reverts to a 0-degree angle while periodically rinsing the drilled area with water.
Note: Being fired and glazed clay, ceramic planters follow the same principle as terra cotta ones, except that ceramics are always glazed with a paint-like substance serving a decorative and waterproof function. This gives them enhanced durability and water-retention capabilities.
3 – Plastic
Plastic planter boxes often have as many drainage holes which can be smaller than 0.25–1-inch (0.63–2.54 cm) since plastic is a durable yet workable material. However, drilling or piercing may no longer be necessary as plastic planter boxes often have drainage holes with removable covers built-in.
Drainage holes can easily be made on a plastic planter box via drills, nails, knives, or cutters. This is because the material is strong enough to support soil and plants but supple enough to be pierced by tools.
Note: Plastic planter boxes are durable and readily available everywhere. They often come with convenient features such as a plastic mesh or a catch plate which further reduce the likelihood of overwatering.
A plastic mesh provides a barrier between the soil and the drained water, serving as a
“water table” collecting the run-off which will eventually be drained. A catch plate works in conjunction with drainage holes to prevent the excess water from spilling on the floor or the furniture.
4 – Fiberglass
Fiberglass planters should have two to four 0.25–1-inch (0.63–2.54 cm) drainage holes along the length of the planter. This can be done with an electric drill fitted with a spade drill bit.
Apply constant downward pressure on the drill starting from the interior to the exterior of the fiberglass planter to make a clean and consistent drill. Dust will accumulate after drilling and should be vacuumed or broomed.
Note: Fiberglass planters are made with fiberglass sheets and resin in a mold. After hardening, they are sanded, cleaned, and painted, creating a durable plant container capable of surviving extreme winter and summer conditions.
5 – Wood
Wooden planter boxes are durable hence they can have multiple drainage holes which can be smaller or larger than 0.25–1-inch (0.63–2.54 cm). Sometimes, drainage holes may no longer be necessary if there are gaps between the planks in the wooden planter box’s construction.
Wooden planter boxes are some of the easiest materials to make drainage holes. Holes can be made through a normal drill electric drill, a spade drill bit is recommended to make a clean drainage hole on wood. In case you do not have a drill that large, just double the number of holes.
Note: Wooden planter boxes offer an excellent combination of aesthetic, durable, and accessible. Wood is generally a durable material that can withstand changing weather conditions. However, durable as they are, they are vulnerable to rotting due to humidity. It’s best to keep them dry whenever possible.
Concrete should have two to four 0.25–1-inch (0.63–2.54 cm) drainage holes evenly spaced along the length of its underside.
Electric drills operating at 18-20 volts with carbide masonry drill bits are recommended for drilling concrete. Concrete, being a durable and hard material, requires an equally strong drill and drill bit for it to be penetrated.
Note: Concrete is the heaviest and most durable material on this list. Due to this, concrete planter boxes are usually reserved for outdoor growing where they can best withstand the elements.
Small metal planters for herbs and flowers require at least one 0.25–1-inch (0.63–2.54 cm) drainage hole at its base. Medium to large galvanized metal containers for vegetables and other plants need four holes or more to ensure adequate water drainage and aeration.
I commonly see gardeners up-cycle old tin cans for tea or turn new metal troughs into sleek but durable planting containers for shade-loving plants. Others prefer keeping it plain and untouched while others add colorful designs to the exterior.
However, there’s one thing that they always make sure to have in metal planters: adequate drainage holes!
Metal is a non-porous material so it holds water incredibly well. Perhaps a little too well, in fact, that your soil is always waterlogged. At the same time, it doesn’t let air deep into the growing medium inside it. That is, unless, you make enough holes at the bottom.
There are two ways for you to do this. First, you can do it using a drill. Secondly, you can also simply puncture drainage holes at the bottom using a hammer and nail.
In case you decide on using a drill, just remember to put the speed on low to prevent the bit from overheating. When the metal is thick or you are using a bit that’s half an inch or bigger, use some cutting oil or lubricant.
Recommended Tools in Making Drainage Holes
Any DIY project requires tools and equipment to make it a reality. Though making drainage holes is a simple DIY project, having the right tools for the job makes for the best results.
An 18-20 volt electric drill is the most convenient way to make a drainage hole for your planter boxes. Though there are manual drills, these pale in comparison to the speed, convenience, and power of an electric drill.
It might seem like an excessive purchase but an electric drill is a fantastic investment in any home. You can use it for gardening, carpentry, or home renovations. It’s an investment that has multiple uses and possibilities.
Modern electric drills come cordless with rechargeable batteries which allow for greater mobility and ease of use while working on a project.
2. Drill Bits
Drill bits most commonly used in making drainage holes in planter boxes are carbide masonry drill bits and spade drills. These drill bits can be acquired in packs with multiples of the same type or in kits with multiple drill bit types.
To make things easier, it is recommended to buy these drill bits in kits. This not only expands your options in a current DIY project but also in future DIY projects down the line.
3. Gloves and Safety Goggles
Gloves are essential to any DIY project, preventing injuries and accidents by providing a protective layer of cloth over vulnerable skin. Goggles prevent particulate matter or debris from damaging the eyes.
Gloves and safety goggles are readily available, often unisex, and with different size options which allows them to be comfortably used.
Drilling produces a lot of dust or debris, especially through concrete. Though not a necessity, a strong vacuum is definitely desirable for health and cleaning purposes.
When looking out for a high-performance vacuum, it is always important to see their recommended surface use. Some vacuums were made for cleaning carpets whereas others were made for cleaning up industrial debris.
Indoor VS Outdoor Use for Planter Boxes
Depending on the circumstances, there are times when a drainage hole might not be desirable. Watering indoors is controlled by the gardener so excess water may not be a problem but a necessity, especially for water-hungry plants.
Additionally, drainage holes may not be desirable in limited indoor growing spaces where water run-off mixed with soil can spill on the floor and furniture. This is especially relevant to condominium units and small apartments where space is a luxury. This problem can be fixed by putting a catch tray beneath the planter to trap the excess water.
However, in outdoor growing conditions, it is always recommended to have at least one drainage hole to allow excess water to seep out. Outdoor plants are susceptible to inclement weather conditions such as seasonal rain which can quickly drown plants.
Which Plants Thrive Without Drainage Holes?
Plants that do best in pots and planters without drainage holes are often succulents, evergreens, and ferns. These plants rarely require much attention and are able to withstand both lack or excess of water.
A list of low care plants that can survive or even thrive in a planter without holes are:
Here are a few examples:
- Chinese Evergreen (Aglaonema)
- Jade Plant (Crassula ovata)
- Cordyline (Cordyline)
- Rough Horsetail (Equisetum Hyemale)
- Croton (Croton)
- Dumbcane (Dieffenbachia seguine)
Planter Boxes in Urban Spaces
Planter boxes can serve as bioretention treatment control in urban spaces by trapping stormwater pollutants in the planter box which will then be filtered and treated by the soil and plants.
Planter Boxes in urban spaces can be integrated into buildings and rooftops where they can have ample sunlight and access to rainwater. The planter boxes are made of concrete or bricks with pipes and underdrains which will remove excess water from the planters.
Concrete and bricks actually retain a lot of heat after prolonged exposure to sunlight. Reducing the surface area of concrete or bricks on the roofs of buildings and replacing them with planter boxes helps dissipate heat and lowers the ambient temperature due to transpiration.
Not only do planter boxes make the grey and drab city skyline greener and livelier but they have noticeable benefits for our environment!
Why can’t drainage holes be bigger than one inch? It is not recommended to have drainage holes that have diameters bigger than 1 inch (2.54 cm). Having a really big hole would only increase the possibility of the growing medium inside the planter falling out. Conversely, holes any smaller than a quarter of an inch (0.63 cm) can easily get clogged and prevent proper drainage.
How to prevent the growing medium from spilling out of the planter’s drainage hole? To prevent the growing medium from spilling out of the drainage holes in a planter, one can place newspaper, paper towel, coffee filter, burlap, or landscape fabric (here on Amazon) at the bottom before filling the container. Some also use fine plastic or steel mesh. But if the holes are of an appropriate size, this shouldn’t be a concern.
Are there other solutions if you can’t put drainage holes in a decorative planter? If one cannot drill holes into an expensive and/or decorative planter, one can practice double potting instead. A smaller pot with holes can be fitted into the larger decorative planter. It is essential to have enough space between the holes and the bottom of the large planter to ensure proper drainage. Water that accumulates must be discarded.
Does adding gravel or rocks at the bottom of a planter improve drainage? No, adding gravel, rocks, or any other chunks of solid material to the bottom of planters does not improve drainage for container gardening. Rather than allowing for drainage, water will instead be collected above it. Maintaining even particle sizes of growing medium better promotes drainage and aeration.
It is recommended to have at least one (1) drainage hole in a planter box to allow excess water to escape. This prevents overwatering and helps aerate the soil.
For more fragile materials, a single drainage hole will do. More durable materials may have two or more drainage holes.
Drilling with the use of an electric drill with the appropriate drill bit is the simplest and most efficient way of making a drainage hole. The recommended diameter of the hole is 0.5-1 inch (1.27-2.54 cm) but this is not an exact science
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