5 Best Practices and Myths About Drainage

As someone who has been in this industry for over 25 years, I’ve learned there are some old habits that should be abandoned, especially when it comes to container garden drainage.  This first blog post is about my habit: the idea that adding gravel or other drainage materials to the bottom of a pot improves drainage. Research from leading agricultural institutions disproves this misconception and provides evidence-based recommendations for proper container gardening.

The Myth of Drainage Layers

For years, gardeners have believed that placing gravel, sand, or broken pottery at the bottom of a container helps improve drainage and prevents root rot. However, research shows that this practice actually creates a barrier that slows water movement. Instead of draining freely, water tends to accumulate at the interface between the soil and the drainage layer, leading to water saturation and increasing the risk of root rot.

Understanding Water Movement in Containers

Water naturally moves downward through soil by gravity. However, when it reaches a denser layer of material, such as gravel, it slows down and pools above this layer instead of passing through immediately. This phenomenon, known as a “perched water table,” traps excess moisture in the soil where plant roots grow, rather than allowing it to drain efficiently.

5 Best Practices for Container Drainage

Rather than relying on drainage materials, experts recommend the following best practices for proper container garden drainage:

1. Use Well-Draining Potting Mix – A high-quality potting mix designed for container plants provides the best soil structure for root health. Avoid using garden soil, which compacts easily and retains excessive moisture.  If you have some large planters, think about using intensive green roof media which has excellent drainage features and is engineered for stability.  The cost is worth the long-term planting success.
2. Ensure Proper Drainage Holes – Containers must have sufficient drainage holes at the bottom. This allows excess water to escape naturally and prevents waterlogging. If the planter can’t be drilled into think about using a liner or pot-in-pot method.
3. Select the Right Container – Choosing containers where drainage holes are either provided or can be drilled.  Avoid using containers without drainage holes unless a drainage layer and a separate inner pot with holes are used.
4. Monitor Watering Practices – Overwatering is a common problem in container gardening. Checking soil moisture levels before watering and adjusting based on plant needs helps prevent excess moisture accumulation.
5. Elevate Containers if Necessary – Raising pots slightly off the ground using pot feet or stands ensures that drainage holes remain unobstructed, allowing water to exit freely.  MVVV provides risers to every purchase of our fiberglass line of planters for this purpose, a complementary product with purchase.

Conclusion

Scientific research clearly refutes the traditional belief that adding drainage materials improves container drainage. Instead, understanding water movement and implementing proper gardening techniques leads to healthier plants. By using well-draining soil, ensuring adequate drainage holes, and practicing mindful watering, gardeners can create optimal conditions for thriving container plants.

For more detailed information, refer to research from Washington State University,

The Myth of Drainage Material in Container Plantings:  “Add a layer of gravel or other coarse material in the bottom of containers to improve drainage” 

University of Illinois Extension,  University of Georgia Extension

What can I do if my container doesn’t have drainage holes?

• Drill Holes: This is the most effective long-term solution if the container material allows it.
• Double Potting: Place your plant in a nursery pot (the plastic pot it often comes in) that does have drainage holes, and then slip that pot into your decorative container without drainage.
  • Important: Regularly check the bottom of the outer container for any accumulated water and empty it.
• Use a Liner: Similar to double potting, use a plastic liner with drainage holes inside your decorative pot. This protects the decorative pot and provides drainage for the plant. Again, ensure you empty any collected water.

How can I prevent the drainage holes from getting clogged with soil?

• Use a Barrier: Place a piece of landscape fabric over the drainage hole(s) before adding your potting mix. These materials allow water to pass through but prevent most of the soil particles from washing out and clogging the holes.
• Avoid Fine, Compactable Soil: Using a well-draining potting mix specifically formulated for containers is crucial. Heavy, clay-based garden soil will compact easily and is more likely to clog drainage holes.
• Elevate the Pot: Slightly elevating your pot on pot feet can help ensure the drainage holes aren’t sitting flush against a flat surface, which could impede water flow.

How do I know if my container has good drainage?

Observing how your container behaves when you water is the best way to assess its drainage:

• Water Should Drain Freely: When you water thoroughly, you should see excess water start to drain out of the holes within a reasonable timeframe (a few seconds to a minute or two, depending on the pot size and soil). If water sits on the surface for a long time or drains very slowly, there’s likely an issue.
• Soil Moisture: After watering, the top inch or two should dry out somewhat before you need to water again. If the soil stays consistently soggy, even with drainage holes, your potting mix might be too dense, or the drainage might still be inadequate.
• Plant Health: Your plant’s overall health is a good indicator, signs of overwatering; yellowing or wilting leaves, mushy stems, stunted growth, brown leaves or flagging leaves.  Check to see if the container is draining as well as pull back on your watering frequency.  Water to the frequency of the soil moisture levels needed.

By paying attention to these factors, you can ensure your containers have the proper drainage for healthy and happy plants.