Container planting: intuition vs. reality

I’m just starting to think about getting my containers planted for the summer and happened to get an email on the topic from a blog reader. John was frustrated with a local columnist’s advice on using gravel in the bottom of the containers for drainage. When challenged, the columnist refuted John’s accurate comments with “logical thinking.” (You can find the posting and comments here.)

Here’s part of the post: “I like to cover the hole with a layer of gravel to improve drainage. Plants need to have their roots exposed to air in the soil to survive and thrive. If the container has no holes for drainage, it will fill with water and drown the plants very quickly. It is better to keep your plants on the drier side than to keep them constantly moist or wet. The big danger in using pots is drowning plants.” Later, he goes on to explain “The potting soil plugs up the drain hole and the water is trapped behind the plug. The layer of gravel creates an area for the water to drain through to escape. The creation of drainage commonly involves a layer of gravel.” This reasoning is part of what he calls “Logical thinking 101.”

As my husband pointed out, this isn’t logical thinking: it’s intuitive. It’s what we think is going to happen in the absence of any evidence. And in this case, it’s wildly inaccurate.

Jeff and I have both discussed the phenomenon of perched water tables in containers as well as the landscape in previous posts and on our Facebook page. The fact is, when water moving through a soil reaches a horizontal or vertical interface between different soil types, it stops moving. Here’s a photo from a very old research paper on the topic:

A layer of silt loam sits above a layer of sand, and water from an Erlenmeyer flask drips in. Intuition says that when the water reaches the sand, it will move more quickly through the sand because the pore spaces are larger than those in the silt loam. But intuition is wrong, as this series of photographs clearly demonstrate. Water is finally forced into the sand layer by gravitational pressure, after, of course, saturating the silt loam.

Intuition has its uses (I am quite proud of my own intuitive powers), but it doesn’t trump reality.

It’s raining, it’s pouring, it’s a good time for a site assessment…

April is turning out to be a soggy month for most of Michigan and our surrounding states.  While most homeowners are inclined to hunker down indoors and keep an eye on their sump pumps on these dark, dreary days; our current run of wet weather is a good opportunity to take a stroll around your property and make some notes.  In particular, note any areas where water is accumulating.

 

Poor drainage is one of the most common sites factors that limit landscape tree and shrub survival and growth.  Sites that retain water for more than a day or too after rains stop are especially problematic.  The challenge with wet areas is we usually wait to plant trees and shrubs until things are high and dry and it’s easy to forget where the wet spots are.

 

There are two primary strategies for establishing healthy trees and shrubs in flood-prone spots.  First, determine if the problem can be corrected.  In some cases homeowners may be able to re-direct water flow from downspouts or other sources to keep water form accumulating in one spot. Again, these kinds of problems are easiest to spot if you go out when it’s raining.  Re-grading the area or installing drain tiles are other options but these are usually require skills and equipment beyond the average do-it-yourselfer.

 

If correcting the drainage issue is not an option, the second strategy is to plant trees or shrubs that are tolerant of flooding.  Plants vary widely in their tolerance of soil flooding and, not surprisingly, trees and shrubs that grow naturally along riverbanks and other low areas are usually the most tolerant.

 


This low spot in my yard  was a good site for a Baldcypress 

There are numerous resources on flood tolerant trees and shrubs on the web.  Two of the better resources are from the Morton Arboretum and from Cornell University.  Please note the Cornell guide is a large (>6 MB) .pdf file.

 


These Michigan holly (Ilex verticullata) I planted a couple of years ago a doing fine even though they are periodically flooded each spring.

Wet areas on your property do not have to be a ‘dead zone’, but establishing trees and shrubs in low laying areas takes some planning.  The first step in the process is assessing your site and identifying the problem areas.  The best way to do this is to put on a raincoat and take a walk in the rain.

Update on root-rotted Cornus kousa

A little more than a year ago I posted a Friday quiz based on a failing Cornus kousa.The answer explained that our landscape has, in part, a perched water table that effectively rotted most of the roots of this poor tree over several years.  Last spring we moved it to a different section of our landscape where we know the drainage is better, and I’ve been monitoring its recovery since that time.


Cornus kousa leaves in 2007


Cornus kousa leaves in 2011

We were gratified to see that the leaves this year are significantly larger than those of previous years.  This tells us that root function has resumed, providing enough turgor pressure to expand the leaves to normal size.  It was especially helpful that we had one of the rainiest springs on record.

Take home lesson:  if a tree or shrub is failing in its current location, it’s worth digging up to see what’s going on.  Bad soil conditions?  Move it to a better location.  Bad roots?  Time to hone your root pruning skills.  But wait until fall to do this.  Transplanting this time of year is the most difficult for plants because of the increased water demands of warmer temperatures and expanding leaves.

Keep Calm and Carry On: Part II

Recently I posted that many of the “rules” that gardeners cling to so tightly regarding tree planting (i.e., dig the planting hole 3 times the width of the root ball, always amend the backfill with organic matter) are probably better considered ‘suggestions’ than rules.  While these practices won’t hurt, there are much better ways to spend time and effort to ensure long-term survival when planting a tree.  Here are the top four:

Irrigate.  No matter how much time and effort goes into the ‘perfect’ planting hole; for most parts of the country, trees that are not irrigated after planting are doomed.  Linda advocates watering in several small sips during the week; I still stick to the old school notion of one long soak per week.  In the final analysis, logistics will probably dictate which approach you use.  Either way, the key is to provide trees with water during the establishment year and even into the second year after planting, if possible.

Mulch.  Organic matter placed properly on top of the planting hole will do more good than organic matter placed in the planting hole.  Study after study demonstrates that mulch conserves soil moisture by reducing evaporation from the soil surface, controls weeds, and moderates soil temperature.  Oh, and that business about wood-based mulches ‘tying up’ or ‘stealing’ nutrients from landscape plants?  Maybe for bedding plants, but not for trees and shrubs.  Our research and other studies indicate that, for the most part, the type of organic mulch makes little difference compared to not mulching at all.  Hence, my motto “Mulch: Just do it.”

Proper planting depth.  Width of the planting hole may not matter, but planting trees too deep is a recipe for disaster.  Burying roots too deep reduces oxygen levels around the roots and starts a series of unfortunate events for the tree.   Find the root collar flare and keep it visible.

Bad move.  The contractor was going to install drain tile but decided not to at the last minute to save money. Ouch.

Right tree, right place. In my experience, the number one reason newly-planted trees fail in the first year is lack of watering and aftercare.  After year one, improper tree selection takes the top spot.  Here in the Upper Midwest, poor drainage and heavy soils take their toll year after year.  Lack of water can usually be addressed, but once a tree is planted in a spot that is too wet for that species, it’s usually a long, slow, and agonizing decline.  And it’s amazing how often people will ignore obvious red flags in selecting trees.  Our Dept. of Transportation recently planted 25 eight-foot B&B eastern white pine, which are notoriously salt sensitive, about 30’ from I-96 at a rest area between Lansing and Detroit.  Predictably, after one winter’s exposure to deicing salt spray all the trees were dead or wishing they were dead.  Right tree, right place.  This ain’t rocket science, folks.