Testing Drainage

Before planting in an area where soil drainage is uncertain, it's highly recommended to test the soil drainage first.

Here's how to test soil drainage:

A percolation test (perk test) is the most effective way to assess soil drainage, even on a slope. To perform a perk test, use a shovel or post-hole digger to dig a hole at least 12 inches deep and 4 to 12 inches wide. If the root balls of your plants exceed 12 inches in height, consider digging the hole up to 18 inches deep. Avoid conducting the test in overly wet soil, as this can create a glazed surface on the hole's sides, leading to inaccurate results. To minimise digging, you can use a planting hole for the test.

Start by filling the hole with water and allowing it to drain completely. This may take a few minutes or an entire day, depending on the soil. This initial step saturates the soil. Once the water has drained, refill the hole and measure the water depth with a ruler. After 15 minutes, measure how much the water level has dropped (in inches). Multiply this value by 4 to determine the hourly drainage rate.

Interpreting the Results

• Ideal Drainage: Soils that drain 1 to 3 inches per hour are suitable for most plants.
• Poor Drainage: Soils draining less than 1 inch per hour indicate poor drainage. In such cases, consider improving the soil's drainage or selecting plants that tolerate wet conditions.
• Excessive Drainage: Soils draining more than 4 inches per hour are very well-drained. These may need amendments or planting with species adapted to dry conditions.

What To Do If Your Drainage Is Poor

If your drainage test shows that water drains very slowly or sits in the planting hole for several hours, action should be taken before planting. Poor drainage can lead to root rot, weak growth, and eventual plant failure.

Below are the main solutions, starting with the most common and effective approach.

1. Improve the Soil by Digging Out and Replacing

In most cases, the simplest and most effective solution is to remove the problem soil and replace it with a better-quality, free-draining mix.

This involves digging a generous planting area (wider rather than just deeper), removing heavy clay or compacted soil, and backfilling with improved topsoil mixed with organic matter and grit.

Pros

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  Immediate improvement to drainage in the root zone
  
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  Gives new plants the best possible start
  
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  Relatively straightforward for individual trees or planting areas
  
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  No specialist equipment required
  

Cons

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  Labour intensive
  
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  Not practical for very large areas
  
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  If surrounding ground is extremely wet, water can still migrate back in
  

This method is ideal when drainage is poor but not severely waterlogged year-round.

2. Raised Planting (For Very Poor Drainage)

If the ground holds water for long periods or sits wet throughout winter, raising the root system above the natural soil level is often more effective than trying to fix the entire ground.

This can be done by:

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  Creating a raised bed
  
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  Building up a planting mound
  
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  Installing sleepers or retaining edges and filling with free-draining soil
  

Benefits

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  Keeps roots above the saturated soil layer
  
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  Provides controlled growing conditions
  
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  Particularly effective for trees that dislike “wet feet”
  

This is often the most reliable solution in heavy clay or low-lying gardens.

3. Installing a French Drain (For Severe Waterlogging)

If water physically accumulates and has nowhere to go, a French drain may be necessary.

A French drain is a gravel-filled trench, sometimes containing a perforated pipe, designed to redirect excess water away from planting areas toward a suitable outlet or soakaway.

When to consider this

• 
  Standing water remains for days
  
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  The garden slopes toward the planting area
  
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  Water collects after heavy rainfall
  

This is a more involved solution and may require professional installation, particularly if connecting to drainage systems or soakaways.

A Final Note

If drainage is only slightly slow, improving the soil is usually sufficient.

If the ground is persistently saturated, raising the planting level or installing drainage infrastructure may be necessary.

This guidance is intended as a starting point. If your site has significant waterlogging issues, further investigation or professional advice may be required before planting.