Soil Depth and Plant Selection

Minimum soil depth for plant growth is a frequent subject of discussion of late and while you cannot search The Plantium based upon soil depth, we thought we’d gather some information to assist in your criteria based plant designs!

Let’s first look at the question at hand. As designers and installers we are asked to create landscapes in all sorts of conditions. Often these conditions are challenging and not necessarily conditions that plants might face in their natural environments. That said, we have all seen these images at some point in our lives…. the beautiful and resilient plant that appears to be growing happily, in what appears to be NO soil depth AT ALL!

 

 

 

 

 

 

 

 

 

For those looking for answers regarding bed construction and just soil depth in general, let’s send you to one of our Plantium brands for some fantastic basic information regarding bed construction in the field.

https://www.provenwinners.com/learn/make-your-bed

Soil depth for OPTIMAL plant growth in more challenging situations such as green roof conditions, large planters, tree wells, etc. is dependent on four main factors. These factors are proper drainage, proper plant selection, plant longevity, and soil depth to plant height ratio. We’ll discuss these factors in this order because the very last issue you should be considering in your plant design is soil depth. If you have not addressed the first three issues in your bed design then regardless of soil depth, plants will not thrive.

Proper Drainage

Whether your planting bed is in the lawn, green roof or contained planters, proper drainage is crucial. While certain plants can survive inundation in a natural setting, standing water or improper drainage will QUICKLY de-oxygenate and compact the soil in contained situations and kill the plants. There are many ways to tackle drainage and all are dependent on the design situation. Working closely with the horticulture team is the best way to find an effective drainage solution.

Proper Plant Selection

Let’s look at those tough little plants growing in the cracks and crevasses! You need to keep in mind the type of plant you are specifying when dealing with challenging planting situations. Plants that grow in these conditions in the natural world tend to be tough, resilient, vigorous, and drought tolerant plants. If you have all the right soil and bed conditions but choose more sensitive plants, it is likely that those plants will struggle. Also, remember that as the soil profile for any plant is constrained so will the plant’s natural habit be constrained. Street trees planted in minimum soil volumes will never reach the full mature height or width that they would under optimal growing conditions.

 

 

Plant Longevity

Planting designs are being designed and installed with a maximum 20 year life span. A controversial statement, perhaps, so feel free to challenge it! It appears long gone are the days of an Olmsteadian landscape that appreciates with time and just barely reaches its full glory at 20 years. Here at The Plantium we vehemently disagree with this trend and firmly believe that criteria based plant selection can reverse this trend. Better plant choices! We’d love to hear your comments on this issue!

All that said, green roof and enclosed planter situations pose different challenges. Small planters and shallow green roof systems need very carefully management to maintain soil health and ultimately plant health. The shallower the soil profile the more quickly that soil will be depleted of nutrients and micro-organisms crucial to the plant’s health. If a refresh of the plant material (and soil) is planned over time, then shallower soil profiles can be a great fit. If the design contemplates trees and shrubs that are intended to grow to maturity over time, then a more significant soil depth should be considered.

Soil Depth

On to the question at hand. Let’s simplify the soil depth for optimal plant growth under container type situations into the following categories. Again, exact soil depth is probably debatable by many so we welcome other thoughts from our expert crowd!

Plant Type Plant Height Minimum Soil Depth
Annuals Any 3”
Turf Grass NA 4” (3” in warm climates)
Perennials <8” 4” (3” in warm climates)
Perennials 8”-16” 6”-8”
Perennials/ Ornamental Grasses/ Shrubs 16”-24” 12”-18”
Perennials/ Ornamental Grasses/ Shrubs 2’-6’ 24”
Shrubs/ Small Trees >6’ 3’ Minimum (should consider overall volume as well)
Trees All Should be calculated on overall volume for each tree and not just soil depth

 

Maintenance

Lastly, on-going maintenance is the greatest issue facing these planting designs. Over-watering is perhaps the most frequent maintenance faux pas committed against our container plants. Carefully working with the maintenance team is critical to head off this issue early on.

Happy Planting Y’all!

 

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