Drainage

Sites with poor drainage can cause patios, walkways, and walls to shift and crack over time; soil drainage affects when and how you can work in different areas (clay soils that drain poorly shouldn’t be worked when wet, as this can cause compaction that lasts for years); some plants need their roots to stay consistently moist, while others need to dry out between waterings. Plus, understanding how water moves through your landscape allows you to design natural drainage solutions like bioswales, rain gardens, or strategic grading.

In this guide, we walk you through conducting a percolation test and a soil composition test. Both of these assessments are essential for successful planning, design, and installation of your landscape. We also cover visual assessments and potential drainage solutions.

1. Visual site assessment
   1. You can collect a lot of information on drainage just by observation. Visual cues that indicate poor drainage include:
      • Areas of standing water that remain wet 24+ hours after rain
      • Water stains on fences, buildings, or other hardscaping
      • Plant stress in wet areas, including yellowing, wilting, or dying vegetation
      • Algae or moss growth in consistently moist spots
      • Depressions and low spots where water naturally collects
2. Soil analysis
   • Percolation test
     • This simple test will give you an idea of how quickly the soil drains throughout your site.
       1. Dig a hole roughly 12” deep and wide in multiple locations around your site. Label them with distinct numbers on landscape flags or a rough hand-drawn map.
       2. Fill the holes with water, then let them drain completely. This first round of water will soak the surrounding soil, mimicking evenly-saturated soil after rainfall.
       3. Refill the holes with water.
       4. Wait for 24 hours.
       5. Measure and record how much water remains in each hole. It’s important to be able to track the holes individually, because drainage often varies throughout a site.
          • Excellent drainage: No water remaining; sandy soils that may need water retention help
          • Good drainage: 2-4 inches of water remaining; ideal for most plants
          • Moderate drainage: 4-8 inches of water remaining
          • Poor drainage: 8+ inches of water remaining; heavy clay or compacted soil that likely needs improvement
          • Standing water: likely requires significant intervention
   • Soil Composition: Jar Test
     • The jar test is an easy way to get a clear idea of your soil type. It’s easy to conduct in tandem with the percolation test: use a jar per hole and label them with the same identifier.
       • Fill ⅓ of a clear jar with soil from your site (use soil from at least 6” deep).
       • Fill the rest of the jar with water, replace the lid, and shake vigorously for 2 minutes.
       • Let the jar settle for 24 hours.
       • Observe the distinct layers that form:
         • Sand settles first (bottom)
         • Then silt (middle)
         • Then clay (top)
         • Organic matter floats atop the water
       • Measure the thickness of each layer to get rough percentages of the composition.
         • For example:
           • Sand = 2 inches = 40%
           • Silt = 2 inches = 40%
           • Clay = 1 inch = 20%
     • Soil types:
       • Sandy soil: 70%+ sand; drains quickly with low water retention
       • Loamy soil: balanced mix that has ideal drainage and retention
       • Clay soil: 40%+ clay; drains slowly with high water retention
       • Silty soil: compacts easily, is erosion prone, and forms surface crust
   • Why perform both tests?
     • The percolation and soil composition tests are necessary complements. There are two important reasons why you should complete both tests:
       • For most kinds of drainage problems, your soil type will inform which solution will be most effective.
         • Any soil type can suffer from poor drainage, but they each require different solutions to improve drainage. For example, clay soils typically need different interventions than silty or rocky soils that drain poorly.
       • When you select your plant palette, you will need to know your soil type(s) anyway. For example:
         • Clay bioswales will drain slowly after rain, so they need plants that can handle prolonged soil moisture– whereas sandy bioswales drain quickly after rain and need plants that can handle cycles of heavy water followed by rapid drainage.
3. Natural drainage solutions
   1. Grading
      • We have an extensive grading guide here that will help you determine if/when grading is the right solution for a drainage problem. 
   2. Soil Amendments
      • Once you have your percolation test and jar test done, you can amend your soil appropriately. Here are overviews of proper amendments for drainage problems in different soil types:
        • Clay soil + poor drainage:
          • Organic matter (compost, aged leaf mold, aged manure)
          • Structural improvers (coarse sand, perlite, small gravel, gypsum)
            1. Never use fine sand– the particle structure creates concrete-like conditions! 
        • Sandy soil + poor drainage:
          • Organic matter 
          • Manual structural improvement (tilling, broadfork loosening, core aeration)
        • Silty soil + poor drainage:
          • Organic matter 
          • Structural improvers (perlite, gravel)
          • Manual structural improvement (deep tilling or aeration)
        • Sandy soil + too-fast drainage:
          • Organic matter
          • Quality topsoil, planting mix, or other more retentive soil types
   3. Strategic plantings
      • Plantings can combat mild-to-moderate drainage problems in the landscape when planned well. Effective plantings will require the information you gathered form the percolation and composition tests. For drainage issues, rain gardens and bioswales are your best solutions; we have extensive construction guides for these elsewhere, but an overview here:
        • Rain gardens collect and filter water in a single, predetermined location, typically a shallow depression  (6-12”) that already exists or that you excavate. 
        • Bioswales collect runoff and disperse it throughout the landscape along intentional routes, usually natural or excavated contours.