When it comes to planting in large pots, many people might be tempted to fill the bottom with bark mulch, landscaping rock, or straw mulch. It is best not to put mulch in the bottom of pots because it will absorb water from the soil, causing it to dry out quickly and harm the plants.
To avoid this issue, a better option is to use river sand in the bottom layer of the pot. This can help to increase drainage and prevent water from getting trapped at the bottom of the pot. By doing this, you can create a healthier environment for your plants and help them thrive.
Why you should avoid putting mulch in the bottom of pots
Large pots can take a lot of soil to fill so it can be tempting to try to fill them with something else to save money. Adding mulch can seem like it will allow the pot to drain but there are the reasons to avoid this.
1. The soil can dry out quickly
Bark chip mulch will absorb excess water from the potting soil. It will also allow large amounts of water to drain quickly out the pot. The plant will dry out quickly and it can quickly kill the plant. Straw mulch, sugar cane mulch and hay will also cause the water to drain out quickly as these dry mulches hold air spaces.
Landscaping rocks will also allow water to drain out very quickly from the pot as large air holes will exist between the rocks. The water will drain and the soil will dry out from the bottom as well as the top. There will also be increased air movement around the root zone of the pot drying out the soil.
2. Plant roots can run out of room
Filling the bottom of the plant pot with a variety of mulches can reduce the space that your plant will have to grow its roots. Quick growing indoor plants like Monstera and Pothos will quickly grow their roots to the bottom of the pot and escape out the holes.
Filling the pot with mulch at the bottom can limit the space the plant has to grow and expand their roots. Allowing your plant to reach its roots down to the bottom of the pot is a fantastic idea and will give you the chance to grow it as large as possible before moving it to a new pot.
3. Rocks can work their way through the soil
While placing rocks in the bottom of a pot at the beginning is a great idea, but rocks will quickly meander through the soil and can end up at the top of the soil. This can send roots growing in a strange direction and can stop them from growing in a natural way.
4. Straw can rot or grow mold
Placing a mulch like straw, leaves, hay or sugar cane mulch can quickly compact and grow mold. The bottom of the pot that is filled with mulch can be the perfect place to grow mold and may even start smelling.
If you have ever placed too much straw into a compost bin without mixing it through thoroughly will know how quickly it can go moldy instead of breaking down aerobically. The same thing can happen in the bottom of the pot and cause problems, can damage roots and create an anaerobic environment.
5. Poor drainage
Mulch at the bottom of plant pots can lead to poor drainage, as water can get caught at the bottom. This can drown the plant roots and cause mold or fungal growth.
6. Lack of nitrogen
As the mulch breaks down in the bottom of the pot it can consume nitrogen from the soil. If there isn’t enough nitrogen in the soil, the plants can suffer from a nitrogen deficiency, resulting in yellowing leaves and stunted growth.
7. Attracts pests
Mulch can attract pests, such as slugs, snails, and insects, which can damage the plants. They can find their way into the bottom of the pot, and can cause damage to the plant roots.
8. The mulch could spread disease
Mulch can potentially spread diseases and pathogens to plant roots if it is not sterile. This can happen if the mulch was made from diseased plant material, or if it was stored in unsanitary conditions.
The introduction of pathogens can harm the plants, leading to stunted growth, yellowing leaves, and wilting.
9. Anaerobic break down can breed fungi
The decomposing mulch can create an anaerobic environment that can promote the growth of harmful bacteria and fungi. When the bottom layer of mulch in a pot becomes waterlogged, it deprives the soil of oxygen, which encourages the growth of fungi.
Fungi thrive in damp environments and can cause root rot, stunted growth, and even kill the plant. This type of breakdown can produce a bad smell, indicating that harmful bacteria and fungi are present.
Mulch in bottom of planters
Placing mulch in the bottom of planters can lead to quick draining planter and dry soil. There are other great options for filling the bottom of a planter box. Starting with large rocks, surrounding them with river sand can be a great way to fill the bottom layer of a planter box.
Placing a layer of geofabric on the bottom of the planter can help to prevent soil or sand from escaping through the holes. This is a great way to prevent mess and staining on pavers or concrete if you have placed your planter in these types of areas.
Can you put mulch in the bottom of flower pots?
Flowers are sensitive to dry soils so placing mulch in the bottom can dry soil out and cause annual flowers to die off quickly. Viola, dianthus, and daisies can die off quickly with dry soil so avoid filling bottom of the pots with mulch.
Placing mulch on the top of the soil can be a great move as this will prevent water loss from the soil as quickly and prevent any weed growth.
For more tips on mulching flowers, check out this video.
Should you put mulch around potted plants?
Putting mulch around the bottom of potted plants can prevent weeds from growing around the pot base. While placing potted plants on a paver or concrete is an obvious way to prevent weeds but if you must place your pot on soil, surround it with bark mulch and prevent weeds.
Placing mulch around the top of potted plants is a good idea. Mulch options can include sugar cane mulch, straw mulch, pea straw or bark mulch. This will help to keep the soil moist and actually improve the soil over time.
The cheapest way to fill planters or pots
The cheapest way to fill large pot or planter is to use old bricks, rocks or logs. Then surround the rocks with river sand to hold the moisture in well while still providing drainage. Making sure you have at least 18 inches of soil is a good guide.
For more on how deep planter boxes should be for vegetables check out my previous article.
Can you put mulch in the bottom of pots? | Summary
Mulch in the bottom of pots can cause the soil to dry out quickly and limit the amount of space that your plant roots will have to grow. For large pots, fill with river sand to allow moisture to stay in the pot for longer while still draining well. Plant roots will even grow into the river sand once they get large enough.
Once the roots reach the bottom, transplant it into a new pot at least a few inches wider and deeper than the previous one. This will give you a great new amount of nutrients for your plant to grow well.
I am an accredited practicing dietitian, experienced gardener and a dedicated cook. I love writing and sharing my experience so you can learn from my successes and mistakes.