What is best to put in bottom of planter for drainage?

Proper drainage is critical for growing healthy plants in containers. Without adequate drainage, roots will sit in water and rot leading to the demise of the plant. The material put in the bottom of a planter before adding soil serves two important functions: drainage and separation. Drainage materials allow excess water to quickly exit the bottom of the pot after watering. Separation materials prevent the soil from washing down and clogging drainage holes. Choosing the right materials for drainage and separation is key to creating an optimal growing environment for container plants.

Why is Drainage Important?

Growing plants in containers is very different than growing them in the ground. In the ground, roots have ample room to spread out horizontally and vertically. Excess water easily percolates down through the soil profile. In a container, the volume of soil is limited. Roots hit the side of the pot and cannot spread out any further. There is nowhere for excess water to drain away. Water accumulates at the bottom of the root ball, surrounding roots in wet conditions. This leads to a lack of oxygen for the roots and encourages fungal root rot diseases. Even if the top of the soil feels dry, the bottom can remain wet for days after watering. Proper drainage is essential to prevent overwatering. The key is to create an environment where excess water quickly drains out of the bottom of the pot after irrigation.

Why Add a Separation Layer?

Along with allowing drainage, materials added to the bottom of a container also serve to separate the soil from the drainage holes. This prevents the fine particles of soil from being washed out of the pot. Over time, soil can migrate downward, clogging up drainage holes and impeding water flow. The separation layer creates a barrier to keep the soil in place. This allows for free drainage through the holes. Without a barrier, roots can also grow out of the drainage holes and anchor themselves. When it comes time to transplant, this makes removing the plant difficult and often results in root damage. An effective separation layer keeps roots growing within the soil environment.

Common Materials Used for Drainage & Separation

Gravel

Gravel is probably the most common material used for drainage and separation in containers. The coarse texture and weight allow excess water to quickly percolate down through the spaces between the rocks. Gravel consists of small stones, pebbles, or crushed rocks. Any type of gravel can be used such as pea gravel, river rocks, or limestone chips. The size of the gravel particles is not critical, but medium sizes between 1/4-1/2 inches work well. Gravel is heavy enough to hold the soil in place to prevent it from migrating down. A 1-2 inch layer of gravel at the bottom of containers is sufficient.

Perlite

Perlite is a popular choice for creating lightweight potting mixes. This inorganic mineral is heat expanded to create white particles that resemble styrofoam beads. Perlite is very porous and absorbs water readily while retaining air spaces. It creates excellent drainage and separation when used at the bottom of containers. A 1-2 inch layer of perlite can be put in the bottom of pots below the soil. It wicks away excess moisture while keeping soil in place. Perlite is sterile and resists compaction. It provides aeration for roots and dries out quickly between waterings.

Vermiculite

Like perlite, vermiculite is a mineral that is heated rapidly to cause expansion. This creates accordion-like flakes that are yellow-brown in color. Vermiculite helps retain moisture and nutrients. It can be used in containers to improve drainage, but retains more water than perlite. Vermiculite compacts over time so it needs to be replaced occasionally. Using a combination of vermiculite and perlite creates a balance of air space, drainage, and moisture retention. A 1-2 inch layer can be added to the bottom of pots.

Sand

Any type of coarse sand can be used in the bottom of containers to facilitate drainage. Sand particles are heavy enough to create separation between the soil and drainage holes. Sands like coarse builder’s sand or horticultural sand work well. The main drawback is that fine particles can still wash through with excess irrigation, potentially clogging the drainage holes. A 2-3 inch layer of sand needs to be added to provide enough weight for separation.

Pebbles

Pebbles are a popular choice for adding style to container plantings. Colorful smooth pebbles or river rocks can be placed on the soil surface as mulch. The same materials can also be used for drainage and separation at the bottom of pots. Pebbles are heavier than perlite or vermiculite so they do not float up into the soil layer. Add a 1-2 inch layer of pebbles below the soil to facilitate drainage. Use larger pebbles or stones and avoid fine gravels or sands which can migrate through the soil.

Shredded Bark

Some growers use shredded bark or wood chips at the bottom of containers. Materials like shredded cypress, cedar, and pine work well to facilitate drainage similar to pebbles or gravel. The coarse, long fibers allow water to pass through but retain some moisture. The main drawback is that shredded bark decomposes relatively quickly. It needs to be replaced annually to prevent clogging at the bottom of the container. Use a 2-3 inch layer.

Landscaping Fabric

Special woven or non-woven fabrics can be cut to size and placed in the bottoms of containers to function as a separation barrier. The fabric allows water to pass through but has small enough openings to block soil particles. Landscaping fabric needs to be cut slightly smaller than the dimensions of the pot. When saturated, soil and roots can still penetrate the fabric so it needs to be replaced periodically. But it works well as a simple bottom barrier.

Packing Peanuts

Polystyrene foam packing peanuts are sometimes used for drainage purposes since they are lightweight and inexpensive. However, some growers avoid them because they can break down to fines that compact over time. They are also not biodegradable. Packing peanuts made from cornstarch are a better option but can also decompose. If using polystyrene foam, make sure to use peanuts and not foam sheets or blocks which do not drain well. Always use new, clean peanuts and add a 2-3 inch layer.

LECA Clay Balls

LECA stands for lightweight expanded clay aggregate. LECA balls are made by heating clay to high temperatures. This causes the clay to expand like popcorn, creating round, porous balls. LECA balls can be used similarly to gravel at the bottom of containers. They wick moisture away from the soil while providing aeration for roots. A 1-2 inch layer of LECA balls is sufficient to facilitate drainage and separation.

Coir Fiber

Coir is made from the fibers found between the husks of coconut shells. It retains moisture like peat moss but is more sustainable. Coir fiber comes in shredded/crimped form or as densified blocks. The long fibers create air space when used as a bottom layer in containers. Coir improves drainage while moderating moisture for roots. It is lighter than gravel or pebbles. Use a 2-3 inch layer of coir fiber or cut blocks to size.

Pumice

Pumice is a porous volcanic rock used in container gardening. The vesicles and cracks create a sponge-like structure that absorbs and holds water well. Yet pumice also has excellent aeration and drainage properties. Use horticultural grades of pumice at the bottom of pots. Pumice allows moisture to drain away while retaining some for plant use. It is heavier than perlite and will not float. Add a 1-2 inch layer in place of gravel or pebbles.

Polystyrene Sheets

Polystyrene foam sheets are inexpensive and easy to cut to size to fit in the bottoms of containers. However, water does not readily penetrate the solid foam so it does not provide drainage. Small holes need to be punched in the foam to allow excess water to drain through. Even with added holes, it is not an ideal system. Avoid using polystyrene sheets for drainage purposes.

Best Materials for Drainage & Separation in Containers

Material	Ratings	Pros	Cons
Gravel	*****	Excellent drainage, allows air spaces, heavy and effective separation layer	Can shift when soil is saturated, heavier to move containers
Perlite	*****	Lightweight, porous structure improves drainage, allows air spaces	Floats up when rewetted after drying out
Vermiculite	***	Holds some moisture and nutrients for plants	Compacts over time, less porous structure
Sand	**	Inexpensive and easily available	Fine particles can clog holes, lacks aeration
Pebbles	****	Allows air spaces, aesthetically pleasing	Small smooth pebbles can shift when wet
Shredded Bark	***	Lightweight, retains some moisture	Decomposes over time, needs replacement
Landscape Fabric	**	Prevents soil from migrating out holes	Roots and soil can penetrate when wet
Packing Peanuts	*	Very lightweight, inexpensive	Can break down to fines, not sustainable
LECA Clay Balls	****	Porous structure improves drainage, permanent	Not as readily available
Coir Fiber	****	Sustainable, moderates moisture and drainage	Can be more expensive
Pumice	****	Porous, improves aeration, retains some moisture	Heavy to work with
Polystyrene Sheets	*	Inexpensive, easy to cut to size	Impedes drainage without holes punched in

Key Considerations for Choosing Drainage Materials

• Pick a material with sufficient weight to create separation between the soil and drainage holes. Avoid lightweight options like packing peanuts that can float up into the soil layer.
• Aim for materials with porous structure that allows air spaces for healthy roots. Solid foam sheets or plastic do not provide good aeration.
• Materials should be permanent or long-lasting. Avoid organic materials like bark or newspaper that break down over one season.
• Inorganic materials like gravel, perlite, LECA clay, and lava rock will not decompose and remain effective for multiple years.
• Consider mesh screens or landscape fabric to combine with other materials and prevent fines from migrating into drainage holes.
• A 1-3 inch layer of drainage material is sufficient at the bottom of containers. No need for a thick layer.
• Arrange materials evenly across the bottom of the pot. Avoid creating drainage channels which can accelerate water flow.
• Replace drainage materials periodically as they can become compacted over time impeding drainage.
• Group plants with similar watering needs together. Mixing drought tolerant and moisture loving plants makes it difficult.
• Consider terra cotta pots which naturally draw away moisture from the soil. Plastic retains moisture longer.
• When reusing pots, discard old soil and refresh drainage layers to restart with optimal conditions.

Examples of Effective Drainage & Separation Layers

Here are some examples of effective drainage systems that can be created at the bottom of containers before adding potting mix:

Gravel + Landscape Fabric

Line the bottom of the container with a fitted piece of landscape fabric to block soil. Add a 2-3 inch layer of clean gravel on top. The gravel provides weight for separation and allows excess water to drain through the fabric and out the holes.

Pebbles + Mesh Screen

Cut a mesh or wire screen to fit inside the pot covering the drainage holes. Place a 2 inch layer of pebbles on top of the screen. The pebbles facilitate drainage while the screen prevents the pebbles and soil from migrating.

Perlite + Vermiculite

Create a 50/50 mix of perlite and vermiculite. Add a 2 inch layer to the bottom of the pot. The perlite provides porous structure for drainage while the vermiculite helps retain moisture and nutrients.

LECA Clay Balls

Fill the bottom 2 inches of the container with LECA clay balls. The porous structure and weight of the balls create an effective drainage system. No need to mix with other materials.

Coir Fiber

Line the bottom of the pot with a coir fiber mat cut to size. The long fibers provide separation along with moderating moisture and drainage.

Shredded Pine Bark

Add a 2-3 inch layer of shredded pine bark to the bottom of the container. The large pieces facilitate drainage while retaining some moisture for plants.

Sand + Landscape Fabric

Place a fitted piece of landscape fabric over the drainage holes to block soil. Add a thin 1-2 inch layer of coarse sand on top which provides weight for separation and allows excess water to drain through.

Materials to Avoid for Drainage & Separation

Some common materials are actually ineffective and should be avoided in the bottoms of containers:

Loose Peat Moss

Loose sphagnum peat moss breaks down when wet into fine particles that compact and clog drainage holes. However, commercially packaged peat-based potting mixes are formulated to provide porosity and drainage.

Straw or Hay

Straw and hay contain organic matter that breaks down quickly and can clump together when saturated. They also tend to float up into the upper soil layer. Use bark fibers instead for better results.

Paper Products

Materials like paper, cardboard, newspaper, paper towels, etc. fall apart when wet. They can block drainage holes and prevent excess water from exiting the container.

Garden Soil

Heavy garden or topsoil is not a suitable material for containers. When saturated, it compacts and becomes dense leading to poor drainage. Use a commercial lightweight potting mix instead.

Moisture Control Gel Crystals

Gel crystals and water-absorbing polymers should be mixed into the potting soil, not used at the bottom of containers. They can restrict drainage when concentrated in one area.

Plastic Sheeting or Landscape Plastic

Solid plastic sheets prevent water from draining out of containers. Even when punched with holes, plastic does not provide adequate drainage or aeration.

Conclusion

The bottom layer inside gardening containers serves the important purposes of drainage and separation. An effective material allows excess water to quickly exit the pot after watering. It also creates a barrier between the soil and drainage holes to prevent clogging. Choosing the right drainage materials can make all the difference in successfully growing healthy plants in containers. Follow these guidelines for creating an optimal system. Monitor plants after installation and make adjustments as needed for ideal growing conditions. With the proper drainage and separation system, container plants will thrive!