Saturday, August 27, 2011

U is for Under

Planting under large trees seems like the perfect solution to bare soil or bare landscapes.  Seldom is any one thing perfect in the plant world.  It comes with it's own set of  "do and don't" and "if and when".

I think expectations might be the number one thing to consider.  It might be impossible to have the lush and colorful ground cover you have in sunny areas. 

Two drawings (taken from the web) show a common misconception about most large tree roots:  The picture at the right, with the deep roots shaped like the tree canopy, is a concept that's seldom correct . 

The drawing below, with the roots near the surface and extending beyond the tree canopy, is how most trees are actually arranged.  

Why is this so important for planting under trees?  We MUST be very careful how we plant and care for those plants or we could kill the tree.  If you assume you can dig a deep hole anywhere, put in your plant, and everything will be good, your assumption could be deadly.

A few don't: 
  • Don't add more soil over the existing understory layer.  The roots will have a lack of oxygen, among other negatives.  The new perennials will send roots vertical and they will not survive either.
  • Don't layer more than 4 inches of mulch over the root area, nor let it touch the tree trunk.
  • Don't plant anything close to the trunk (where there are large roots). 
  • Don't use a weed killer on the grass or other vegetation under the trees.
  • Don't plant annuals as you risk disturbing the roots every spring.
  • Don't hammer in weed barrier edging or lay heavy stone edging on top of the tree's root system.  It will eventually cut or compress the roots that are needing that expansion to grow the tree.
  • Don't use gravel mulch under trees as the weight of the stone isn't healthy for the roots. 

A few do:
  • Remove grass or other vegetation by hand pulling. 
  • I like to use a dull hand spade to gentle make a hole for the perennial.  Do this a little at a time in case you run into a root.  Think of it as heart surgery, where you use your fingers to make sure you don't cut anything necessary for sustaining the life of the tree.
  • Plant only small container plant starts.  This allows for the plant to be planted between roots.
  • Don't plant perennials that grow large and will compete with the tree for nutrients and beauty.
  • Water deeply and often the first year as the new plant strives to establish roots.  The tree's established root system has the advantage of being able to take up water and nutrients more easily.
  • Fertilize the new plants at least monthly to help them get the nutrients until they can establish their own root system.
  • Add a one inch layer of crumbled dry horse manure.
  • Add no more than four inches of organic mulch when done. 
  • I don't use a fabric weed barrier because it will inhibit (to some degree) water flow to the roots - something in dire need when establishing perennials.
  • Check out the light during the day under your tree - some might have more sun than shade loving perennials might tolerate.
  • Not everything will grow under walnut trees.
Other realizations:
  • A ground cover strong enough to compete with the tree roots is also hardy enough to be an aggressive spreader.  Even a shade perennial will reach for the sun.
  • Adding ground cover, edging, and other elements at the same time you plant a tree is easier and healthier on the tree than planting under a large established tree.
  • Planting ground cover requires less plants because they will spread and cover the area.
  • Add a few spring blooming bulbs in the same hole as the new perennial to give pretty spring color plus eliminate the need for more holes.
  • Root damage is condemning a tree to a slow ugly death. 

There's nothing wrong with no perennials growing under trees. 
  • A neat layer of mulch.
  • Pots of shade loving flowers.
  • A piece of sculpture or garden chotsky.
  • A garden bench or seating
  • Bird feeders and baths

“A society grows great
when old men plant trees
whose shade they know
they shall never sit in.”
Greek Proverb    


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