Some plants are sensitive to excessive moisture around their root zone, and stagnant water at the bottom of the pot can cause root rot, so it’s sometimes necessary to increase drainage when growing plants in pots. There are 6 materials listed below to improve drainage in pots, such as Perlite, Coarse Sand, Gravel, Rocks, Vermiculite, Activated Charcoal, etc. Other factors that help include using the right techniques for planting, selecting the right container based on the plant’s size, watering appropriately, and omitting the addition of drainage materials.
Table of Contents
Best Material For Improving Drainage In Pots
1. Quality Potting Mix
A perfect soil mix makes the best and simplest planter drainage material.
Professional gardeners use specialized potting soil blends that offer the ideal balance of moisture retention and drainage to grow plants with various watering requirements.
Succulent soil mixtures will differ greatly from flower potting soil.
- Avoids the need for additional pot drainage material
- Provides a blend of drainage and nutrients for better plant growth
- Easy to use straight from the bag – no extra work mixing
- Prevents soil compaction, which improves drainage and aeration
- Can be expensive, especially if you have an extensive container garden
- May not be practical if your pots already have soil inside you want to use
- The soil may still retain too much moisture for certain plants or inside specific planters
According to experts, if it fits into your existing gardening system and your budget, filling a pot with a well-draining potting mix is the best option.
What About Debris That Enters Drain Holes?
Cover the drainage holes at the base of the pot with a coffee filter or a piece of window screening if you’re concerned that dirt will leak out when you water them. After the pot has settled from the initial watering, potting soil rarely escapes from drainage holes.
Perlite is a white, lightweight, highly porous material which is produced by rapidly heating volcanic silicate rock to high temperatures above 870°C (1,600°F), which causes the water in the perlite to turn into gas, causing the heat-softened mineral to expand like popcorn, by 4–20 times its original volume.
Depending on the grade, expanded perlite has a bulk density of between 30 and 150 kg per square meter (or 2 and 9 lb per square foot). It comes in different grades ranging in size from 3 – 6mm (1/8 – 1/4″) in diameter.
In addition to being a common hydroponic growing medium, it is also widely used in potting media, greenhouse growing media, and nursery propagation applications. There are fine grades of perlite that can be used to more easily fill very small containers for the production of seedling plugs.
- increases drainage and aeration.
- Non-harmful, sterile, and odorless.
- Chemically inert, pH neutral (7.0–7.5), lacking pH buffering capacity, devoid of any mineral nutrients, and almost devoid of CEC (cation exchange coefficient), it is unable to hold nutrients.
- Over time, won’t compact.
- Water is only retained on the surface and in the spaces between particles due to the low water-holding capacity. Has “closed cell” pore structure, so pores don’t absorb or hold water.
- When compared to coarse sand, which improves drainage but adds weight and reduces aeration, lightweight potting medium reduces weight while increasing drainage.
- can be sterilized using steam heating.
- Moderate cost.
- tends to float to the top of the potting soil when watered.
- When dry, dust is very dusty and dangerous to breathe in. Use a dust mask while handling the product to reduce health risks.
- Before combining with other ingredients, it must be moistened to prevent dust.
- may have fluoride levels that are potentially toxic to plants that are sensitive to fluoride. Keep the pH above 6 to prevent fluoride toxicity issues, and avoid using commercial phosphate fertilizers that contain fluoride (like superphosphate, diammonium phosphate, and ammonium nitrophosphate), which you shouldn’t be using in the first place because they’re synthetic, not organic-certified, and really bad for your soil!
- When the pH is low, can release toxic levels of aluminum into solution; this issue can be avoided by maintaining the pH above 6.
3. Gravel Or Rocks
Every gardener has heard that filling in drainage holes at the bottom of a pot with rocks or gravel prevents soil from falling through while still allowing excess water to drain from the soil above.
In reality, this notion has been proven false, as scientific research shows using coarse material at the bottom of planters causes water to stop flowing and collect on top, keeping the soil wet.
Beginner gardeners who are unaware of this problem struggle with root rot and dead plants as a result of too much water in the soil. So, if all you have is gravel or rocks, is there a way to improve soil drainage using those instead?
- A layer of rocks on a planter bottom is fine if you perch another plastic pot (with drain holes) holding the soil and plant on top, which increases humidity at plant roots but keeps water draining
- Superfine gravel mixed throughout your potting soil improves aeration and drainage while reducing compaction
- Gravel is free or cheap
- A layer of gravel or rock at pot bottoms forms a barrier that prevents natural water drainage
- Increases overall weight of the pot, making it harder to lift and move
- Roots can’t grow into gravel, which can restrict root development
4. Activated Charcoal
When considering materials for planter drainage, most gardeners never consider activated or horticultural charcoal. This material even performs admirably in pots with absolutely no drainage holes, which are typical for indoor plants.
- Inexpensive to purchase and easy to use
- Absorbs excess moisture without getting soggy so that roots won’t rot
- Has natural antimicrobial action, which reduces fungal, disease, and pest issues
- Can be mixed into soil or used as a base layer
- Pulls harmful toxins from the soil or water, so plants are healthier
- Can be dusty – Good idea to use gloves and use them in a well-ventilated area
- Do not make the error of substituting grilling charcoal (please do not!)
Vermiculite is a lightweight, highly porous material, consisting of glossy flakes that vary in colour from dark gray to sandy brown, which are produced by heating chips of the layered mineral mica to high temperatures of around 800 – 1100 °C (1,472 – 2,012°F), which causes the laminated, plate-like structure to expand like an accordion, resulting in a highly porous lattice structure with beneficial aeration and water-retention qualities.
It typically comes in four different grades, with grade #1 being the coarsest and grade #4 being the finest in terms of particle size. Expanded vermiculite is very light, with a bulk density ranging from 64 to 160 kg/m (4 to 10 lb/ft), depending on the grade.
It helps potting media retain more moisture and nutrients. While using the coarser grades for potting media, the finer grades are used for seed germination and to topdress seedling flats.
Vermiculite is sold in garden centers for horticultural use only; vermiculite sold for industrial or construction uses should not be used for gardening.
- increases drainage and aeration.
- Non-harmful, sterile, and odorless.
- Excellent pH buffering ability.
- It can hold soil nutrients and release them gradually because of its moderately high CEC (cation exchange coefficient, 2 to 2.5meq/100cc).
- Potassium, magnesium, and calcium are present and gradually become available to plants.
- In contrast to perlite, this material is highly absorbent and has an extremely high water holding capacity.
- Easily compressible, should not be compacted or pressed, especially when wet, as this will damage its structure and reduce its capacity to hold water and air.
- less durable than perlite and coarse sand.
- The particles in the finer grades, which are used to fill seedling plug trays, are too small to hold much air or water for growing roots.
- Depending on where the mine is located, the pH can range from slightly alkaline to extremely alkaline. The majority of the vermiculite produced in the US has a pH of 6.3 to 7.8, which is neutral to slightly alkaline, whereas the vermiculite produced in Africa can be quite alkaline, with a pH of around 9.
Vermiculite And Asbestos Contamination
In the 1990s, vermiculite’s possible contamination with fibrous tremolite asbestos raised health concerns. The vermiculite mine near Libby, Montana, which was started in the 1920s, was the biggest and oldest vermiculite mine in the US. From 1925 to 1990, it was producing more than half of the world’s vermiculite, which was later discovered to contain asbestos and fibers that are similar to asbestos. In 1990, asbestos contamination caused the site’s mining operations to be suspended.
The majority of mines today closely monitor their operations to prevent issues with asbestos contamination in commercially available expanded vermiculite as a result of this incident and the attention it attracted, which led to a significant lawsuit against the mining company. As a safety measure, it is suggested to keep vermiculite moist while using it to reduce dust and to wear a dust mask to prevent inhaling any dust from the substance.
The Differences Between Vermiculite And Perlite
For some applications, vermiculite and perlite are interchangeable because of the larger pore space that their particles provide, which improves drainage while reducing soil weight.
They do have some differences, though, and these will determine whether one is used more frequently than the other.
- Vermiculite is made up of an expanded, plate-like structure with air spaces between layers that enable it to absorb water.
- Perlite has a “closed cell” structure without open pores, and cannot absorb water.
Cation-exchange coefficient (CEC)
- Potassium, magnesium, and calcium are examples of positively charged nutrients that vermiculite can bind and then gradually release.
- Nutrients can’t be bound by perlite.
- Vermiculite has a pH range of slightly alkaline to very alkaline and a pH buffering capacity that keeps the pH stable.
- The pH of perlite is neutral, and it has no capacity for a buffer.
- With repeated digging, the soft, compressible vermiculite particles will easily compact and lose their beneficial properties.
- Because perlite is made up of hard, brittle particles that cannot be compressed, it can withstand repeated digging without breaking up completely.
6. Coarse Sand
If you use dirt from your yard that has a high clay content, sand can improve drainage in pots with dense soil. Use sand with a coarse texture rather than ultra-fine silica sand, which will only exacerbate your drainage problems.
Sand has a right way and a wrong way to be used in potted planters, so you need to be careful when using it to improve drainage.
- Inexpensive and readily available
- Loosens soil texture, which improves aeration and drainage
- The finer texture doesn’t create a water barrier like rocks or pottery shards as the base of pots
- An inch or two at the bottom of a planter with no drain holes can lift roots out of pooling water
- Too much sand in the potting mixture can drain off the water too fast, leaving plants thirsty
- Sand is heavy and may settle toward the bottom of the pot during watering, so yearly soil mixing is necessary
- Sand with a fine, rounded texture can inhibit drainage by filling in natural air gaps in the soil
Drill small drain holes about 1/2 an inch up the bottom of the pot’s perimeter if you want to use sand as the base layer for your pots.
Holes in this location allow excess water to drain out of the sand to prevent root rot while maintaining sufficient moisture in the pot to provide humidity to the roots.
Other Amendment Materials Which Can Be Used To Improve Drainage In Pots
All of the inorganic (non-carbon based, not made from living matter) soil amendments that have been discussed thus far increase the pore size between media particles, resulting in larger air spaces that reduce the capacity of the media to hold water and enhance drainage and aeration.
There are many other inorganic materials that can be used for the same purpose, such as:
Pumice – extremely porous igneous volcanic rock naturally expanded by gases in the molten rock. the volcanic glass that has been rock-foamed and has so much air inside of it that it floats on water.
Scoria – very porous igneous volcanic rock naturally expanded by gases in the molten rock. a dense, porous basaltic lava that sinks in water that has pores that are less than 1 mm in size, much smaller than those found in pumice. Even though it can also be black or dark brown, the most typical color of scoria (lava rock) used in landscaping is red. The red scoria is actually black at first, but during a volcanic eruption, the iron in the rock is oxidized (chemically rusted), turning it red. The name scoria comes from the Greek word for rust, which is σκωρία, skōria.
Pea-gravel – small, smooth, rounded pebbles up to the size of a large pea, shaped naturally by exposure to running water, or through a tumbling process for polishing. For top-dressing the soil in water plant pots in ponds and water gardens to prevent clouding the water, aquarium gravel is typically smaller in size.
Clay balls – used extensively as a hydroponic growing medium, these are not actually balls of clay, but rather small pumice balls coated with a later or clay. The cost of using an amendment material is quite high.
The only issue with mixing rocks into potting soil is the obstacle they present when digging, which is similar to using a shovel in rock-filled soil. This shouldn’t be too much of a problem if the potting mix won’t be frequently dug into.
Some of these materials might also contain fine rock dust, which needs to be removed by washing in order to prevent it from clogging the potting mix’s air spaces. To do this, simply place the rocks in a pot with drainage holes and hose them down with a high-pressure water jet. Put a piece of shade cloth or flyscreen material inside the pot first to prevent the rock from washing out if the drainage holes in the pot are too large.
How To Test Drainage In Potting Mixes
Testing a potting medium to determine whether drainage improvements have been made is likely worthwhile after the changes have been made.
Here is a simple test to determine how well a potting mix drains:
- Fill a pot with 1 liter (approx 1 US quart) of dry non-amended potting mix.
- Fill a second pot with 1 liter (approx 1 US quart) of dry amended potting mix.
- Each potting mix pot should have 500ml (or 2 US cups) of water in it.
- After a few minutes, count the amount of water that drains out and compare the two.
After a few minutes, roughly half the water, or 250 ml (or about 1 US cup), of the mixture should drain out if it is draining properly.
More water should drain out of the pot containing the amended medium if the addition of the amendment to the potting medium has improved drainage and decreased water retention.
How Different Pot Materials Can Affect Drainage
Without altering the potting medium, how can we improve drainage?
By altering the pot! A pot’s construction material has a significant impact on drainage.
Unglazed terracotta pots are porous and will wick water away from the potting medium, and are therefore ideal for plants that prefer better drainage.
Plastic pots only lose water from their drainage holes at the bottom and tend to retain more moisture, making them a great choice for plants that prefer more moisture.