Soil Depth and Plant Selection

Dive into The Shallow End

As designers and installers we are tasked with creating landscapes in all sorts of conditions. Often these conditions are challenging and not necessarily what plants might prefer in their natural environments. Soil depth is one of the most common challenges we face. From containerized plantings on a pool deck, to street trees or green roofs, almost every designer will eventually need to choose plants for a constrained location.  This article will give you some insight into creating successful plant designs in tight places!

The definition of limited soil volume!

Talk about limited soil depth!

Making sure your plants have enough room to root seems like it would be the primary concern when creating a design with restricted root space. However there are really multiple factors you have to consider before you even get to soil depth!

Proper Drainage

Planting areas with restricted soil depth or restricted soil volume are notoriously prone to drying out quickly, but don’t be fooled into thinking you don’t need to address drainage! Whether your planting bed is in the lawn, green roof or a planter, 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 your oh-so-carefully selected plants. There are many ways to tackle drainage and all are dependent on the design situation. If you are working on a new install, drainage is first and most important thing to get right, as is can be difficult and costly to fix down the road. If you are working in an area with existing poor drainage, find solutions before considering any next steps!

 

Soil Health and Longevity of the Design

The anticipated lifespan of landscapes varies considerably. Before you choose plants, consider how long you anticipate your design persisting before a “refresh.” Are you creating a container design that will be replaced every year or even every season? Are you planting trees and shrubs you want to see mature over time?

Planting areas with restricted root space require monitoring of soil health. The more restricted the soil volume, the more quickly that soil will be depleted of nutrients crucial to the plant’s health. However if a refresh of the plant material (and soil) is planned to occur frequently, then shallower soil profiles can be no problem. If the design contemplates trees and shrubs that are intended to grow to maturity over time, then a more significant soil volume should be considered to promote long-term plant health.

Proper Plant Selection

Let’s look at those tough little plants growing in the cracks and crevasses! 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 sensitive plants, it is likely your design will still struggle without intensive care. And don’t forget how soil volume can affect plant hardiness. Plants with reduced soil volumes, especially container plantings, are much more susceptible to larger swings in soil temperature. If you are planting long-lived plants in containers it is best to pick plants at least two zones hardier than you normally would.

Don’t forget, The Plantium can help you quickly select plants for even the toughest planting project!

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Soil Depth

And now we finally come to soil depth.

We have all seen the beautiful and resilient plant that appears to be growing happily, in what appears to be almost no soil volume. However, plants grown in-situ from seed are are much more successful establishing themselves in challenging growing areas than transplants.  Imagine going from a pot in a comfortable nursery to being blasted by sun and heat in a roof deck! To have you transplanted plants thrive, follow these basic rules of thumb for soil depth.

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

If you are looking for more information on soil depth and bed construction, check out this great article by Proven Winners.

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

Maintenance

Lastly, on-going maintenance is the greatest issue facing these planting designs. In addition to soil replenishing mentioned above, containerized plants can also suffer from large swings in soil moisture. Because they are prone to drying, too much irrigation is often applied, and over-watering is perhaps the most frequent maintenance faux pas committed against plants with restricted root space. Carefully working with the maintenance team is critical to head off this issue early on.

Happy Planting !

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