Wednesday, January 16, 2013

Planting Holes and Amending the Backfill Soil

To Amend or Not To Amend, that is the question

Cornus kousa
CrabappleLandscapExperts are practiced experts with Georgia red clay, the native soil type of metro-Atlanta.

Although some property managers and HOA Reps may wish to resort to the heavy use of soil amendments to ‘improve’ the native soil, our experience agrees with the horticulture specialists at the University of Georgia Extension Service and experts across the U.S., who do NOT recommend heavily amending individual planting holes.

Research has shown there are 2 reasons that plants in individually-amended planting holes do worse: 

girdling roots 
Reason 1: if the back fill soil has a lot of pine bark or compost added to a moderate-sized planting hole, the newly planted shrub follows the path of least resistance. Instead of the roots growing straight out of the dug hole and into the surrounding soil in all directions, they continue to circle around and around in the “fluffy soil” inside the planting hole. To my mind, the roots never achieve the "jump to hyperspeed” (like the Millenium Falcon in Star Wars) and are contained inside the hole.

What results is a restricted, circular root system that remains confined to the limited extent of the planting hole and can eventually girdle the trunk, killing the plant. Even though planted in open ground, the plant effectively becomes “pot bound” and is more sensitive to both drought and excess rainfall.

Reason 2: excessive soil amendments added to a small area upset the moisture balance between the planting hole and the surrounding soil. Roots need oxygen too! In a wet year the organic amendments soak up and retain water like a sponge in the planting hole, absorbing and holding too much moisture and causing “wet feet” as when the roots to stay too wet and rot or drown without oxygen. This is known as the "bathtub effect". 

prepared flower bed
Entire Flower Beds, Lawns or Gardens
Now, these soil amendment considerations do not apply when Crabapple prepares annual beds, an entire lawn or a complete garden area, but only when individual holes are dug for woody ornamentals and trees. In the case of entire beds, there is no problem with water relations or moisture balance in the root zone of the plants because they experience uniform conditions.

Diagram courtesy of the City of Bowling Green, KY 
LandscapExperts Best Practices 
Optimal growth is achieved when a large, wide (but not deep) planting hole is dug, and the backfill soil is well worked and aerated. Our planting holes are two or three times the diameter of the container or root ball, and we take care that the top of the roots are level with the soil surface and not sunken. We backfill with the same soil removed from the hole but only after breaking apart clods, removing stones and debris, and smoothing out the texture. Slopes and drainage are also considered. In compacted clay soils a large planting hole with loose backfill soil is essential for proper plant establishment.

Finishing Touches
CrabappleLandscapExperts thoroughly water-in the newly planted woody ornamentals. We then place organic matter on top of the surface as a mulch, rather than in the planting hole as an amendment. Learn more about the benefits of mulch in our January 9, 2013 blog. 

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