Growing Concerns 246: Raising soil levels around woody plants

We recently bought a house and want to raise the beds around the house where the soil has compacted and eroded over the years. We would like to raise the level of the beds about eight inches to help keep the water out of the basement and improve the look of the beds. The smaller plants in the beds can be raised easily to the new height, but we are concerned about some older plantings which cannot be raised.

We have a very old climbing hydrangea that covers the front of our home and do not want to risk damaging it. Many of its surface roots are becoming exposed. I understand most plants do not like to have their roots and stems buried deeper, but can the hydrangea be buried a few inches without damage? Are the roots supposed to be completely under the soil or does the hydrangea prefer the roots to be very near the surface?

We would also like to increase the size of our garden to include the base of a couple of trees. Would it hurt our magnolia, birch or oak to raise the soil and plant at their bases? If we do begin this project, when is the best time to do it, now or another season? We would rather forgo the flowers than harm the trees, so your input is appreciated. - K. -

We can raise soil over roots, if we know how roots function and where younger and older roots are.

Most woody plants form shallow root masses -- most are less than eighteen inches deep -- that are very wide. An established climbing hydrangea may root outward farther than its longest vining branch reaches upward.

We say tree roots "surface" as they age, but this isn't so. "Surfacing roots" grew there from the start to exploit the abundance of nutrients and oxygen in upper soil levels. Roots become woody and stout with age, like branches, so a root that began life two inches deep stays there but gradually adds wood -- when it's six inches in diameter its upper surface breaches an inch above ground.

Asking for trouble to bury roots at dripline and beyond

A root's needs change over time. In thin-walled youth, it absorbs water and nutrients directly. Once woody, it's not a primary collector but a conduit for what's collected by root hairs at its tips. It's also tougher, able to handle pressure changes. As soft youngsters, roots can't tolerate extra weight, nor can microorganisms nearby -- millions of microscopic animals and plants that exist in each teaspoon of healthy soil. To bury such tiny creatures and delicate structures with even an inch of extra soil is like plunking a mountain down on us. The change in pressure and oxygen levels would kill us, as it kills them.

If a young root isn't crushed outright, it still faces starvation rations as fungi, bacteria, and other soil dwellers perish. The nutrient production they represent falls off as they die.

Most feeder roots are at the root mass perimeter -- we figure this area begins at the drip line, the furthest reach of the branches. Since woody roots are less sensitive to changes and can tolerate five or even ten inches of extra soil if it's loose and fluffy, it's fairly safe to make a raised bed that involves only woody roots -- in the area from drip line in toward the trunk.

If feeder roots must be buried, cover only one-third of their area in any one year. Remaining roots, if kept well watered and fed, can make up for reduced production by the buried roots. Alternatively, cover all the roots but only lightly and build up gradually, year by year.

Don't bury the trunk

Don't confuse roots and trunks. Trunks are not adapted to life under ground. Slope a raised bed down to the original grade around a trunk, or build a barrier to keep soil away. A few shrubs and trees -- especially those adapted to life on a river floodplain -- can tolerate soil against bark, but most buried trunks develop serious problems such as collar rot.

Early fall may be best for raising beds over tree roots. Then older, starch-storing roots are more important than feeder roots. Many feeder roots will be shed and replaced before spring, so the tree has a chance to reposition its foraging roots in the new upper crust.