Thursday, August 30, 2012

Mosquito Repellent Plants

Mosquito Repellent Plants | Garden Design#gallery-content#gallery-content

Above is a link to an article from Garden Design Magazine on mosquito repelling plants that I thought would be of particular interest to readers this time of year.  To build on the plant species recommendations noted in the article, I have the following suggestions:

Of the plants discussed in this article, only Catmint and Lavender are perennial in the Midwest region (they go dormant in the fall and regrow every spring).  Of these two, I would recommend Catmint more due to its drought tolerance and ability to “thrive on neglect.”  Lavender requires more attention to get established because it can be finicky about soil and moisture conditions, factors that the “weekend warrior” homeowner may not have the time or expertise to mitigate.

Citronella Grass, Rosemary, Basil, and Scented Geraniums are annuals in the Midwest region (you’ll have to replant them every year) since they usually won’t survive our cold winters unless potted and brought indoors.  However, they are excellent candidates for container gardens and planter pots where they can be employed as versatile, mobile mosquito control – just move the pots around your porch, deck, or patio as needed.

Sunday, August 12, 2012

Meatballs and Flattops – for the kitchen and barber shop, not your landscape!

Tightly-sheared geometrically groomed shrubs are a common sight throughout commercial and residential landscapes.  I disdainfully refer to them as “meatballs” since the default shape many people force upon shrubs is the sphere.  However, you can find cubes, cones, spirals, cylinders…just about any shape that a shrub doesn’t want to be.

The horror...
...the horror!
Densely branched shrubs with small leaves, such as Yews (Taxus spp.), Boxwood (Buxus spp), and Barberry (Berberis spp.) are the usual victims, however, anything with leaves within arm’s reach is susceptible.  Not that sheared shrubs do not have their place in certain landscape applications, however, shearing is done too often by default rather than by design.
Shearing shrubs for an impeccably maintained aesthetic can be traced to many historical roots.  Topiary practices of training and pruning shrubs into distinguishable and/or artful shapes can be traced as far back as Roman Empirical villas and Japanese Zen Gardens.   European Renaissance Gardens of the 15th and 16th centuries also incorporated strong geometric forms with sheared shrub hedges and specimens, most notably French parterre gardens such as the Gardens of Versailles.  Key design elements of these gardens were symmetry and formality.
Many of the everyday contemporary landscapes where I observe meatballed shrubs would not be classified as formal and symmetrical.  Rather, the shrubs are sheared because (1) the owner / maintainer is trying to keep the shrub from getting “over-grown” or (2) because shearing is perceived as quicker and easier than properly pruning or (3) they owner don’t know any better.

Here is an example of a row of yews planted with the intention of screening and softening a large expanse of building wall.  However, that original design intent has been lost due to the unnatural and stunted forms forced on the shrubs.

Here is the same view modified to show what could have been if the shrubs were allowed to take on their natural form and habit.  The man-made, monolithic brick expanse is softened and more effectively screened by the natural forms.
Shearing a shrub to keep it from becoming “over-grown” is not a matter of proper maintenance but a way to deal with having put the wrong plant in the wrong place.  If a shrub has to consistently be cut back in order to keep from blocking windows, views, or pathways, then the shrub variety was not wisely chosen for that spot.  A smaller dwarf variety would have been a better choice.
Shearing a shrub because it is “quicker and easier” than selectively pruning limbs is a misconception involving looking at maintenance in terms of intensity rather than frequency.  How long does it take to shear a shrub? A few minutes, in most cases.  How long does it take to prune a shrub (selectively cutting individual branches to accentuate the shrubs natural form)?  That depends on the size of the shrub, but more than a few minutes in any case.  However, how often do you have sheer a shrub to maintain that perfect meatball or flat-top?  Multiple times per growing season.   How often do you have to prune a shrub?  Typically, once per year, if that.  Wouldn’t you rather invest a little more time just once a year instead of an on-going maintenance hassle throughout the growing season?
The final reason shrubs are commonly meatballed is a matter of the owner’s taste.  Some people just want a well-kempt and tidy appearance for their shrubs.  My first question would be: does the rest of the landscape warrant manicured shrub shapes?  Is your landscape strongly based on formality and symmetry?  If not, those meatballs are going to stick out like sore thumbs. 

A good example of a mass of yews with natural form.
image courtesy of Oregon State University Department of Horticulture
A speciman boxwood left to grow into its natural form.
image courtesy of Oregon State University Department of Horticulture
Naturally formed Witherod Viburnum
image courtesy of Oregon State University Department of Horticulture

I actually had a client tell me that the only reason he wanted his shrubs meatballed was so he could tell that maintenance crews had been there to ensure he was getting his money’s-worth.  I don’t know how to argue with that.  Hopefully, though, I’ve persuaded and informed some readers to design properly, save time and money, and save your shrubs.

Tuesday, July 17, 2012

Retaining Wall Basics - Part 2

My last post discussed the basic forces a wall must resist when retaining grade: the weight of soil beyond the angle of repose and hydrostatic pressure.  Different means of resisting these forces are required based on the material used to construct a retaining wall.

Wood Timber Walls
Wood Timber Wall Section
Wood timber retaining walls achieve structural integrity partly from the spikes used to tie each course of timber together. However, spikes alone are typically not enough to keep a wood timber wall from failing. The addition of deadmen is what provides true stability to the wall. A deadman is a wood timber that is tied perpendicular into the face of the wall and runs horizontally back into the retained grade, then has another timber tied to the opposite end that is positioned parallel to the wall. The deadman must extend a sufficient distance into soil within the angle of repose in order to maintain a stable “grip.”  Wood timbers are a relatively affordable building material and don’t necessarily require professional installation.  However, proper design of these walls is critical to determine the necessary length of deadmen.  Another disadvantage to wood timber walls is their susceptibility to rot, even if constructed with treated wood.
Precast Concrete Block Walls
Precast Concrete Block Wall Section
Precast concrete block walls, also called segmental retaining walls, get their structural integrity from structural reinforcement and geogrid.  The most common forms of reinforcement lips cast into the blocks or metal pins that tie each course of block together, depending on the preference of the block manufacturer.  When installed properly, these lips or pins establish a batter to the wall for structural  support.  This batter may be all that is needed for shorter walls.  However, for walls that are taller or have a surcharge (additional live load) placed on them, additional support is provided by geogrid.  Geogrid essentially performs like a deadman in a wood timber wall: it connects to the vertical wall and extends horizontally through the retained grade into stable soils below the angle of repose to form an anchor.  The advantages of segmental retaining walls are a relative low cost when compared to other wall systems and ease of installation.  Disadvantages of concrete block walls are their modular man-made appearance; however, many manufacturers are developing units with more convincing natural shapes and textures.
Stone Boulder Walls
Stone Boulder Wall Section
Stone boulder walls retain soils by means of the sheer mass of the stones.  The structural integrity and height of a stone boulder wall is directly proportional to how large of stones used to construct it.  These walls provide a natural and “solid” appearance and will last a lifetime or more if designed properly.  However, they require heavy machinery to install a wall that can achieve a significant height, and calculating design requirements can be complicated due to wide variability of stone sizes, shapes, and availability.
Dry-Laid Stone Walls
Dry-Laid Stone Wall Section
Dry-laid stone walls typically use smaller, flat, linear stones stacked in courses as opposed to randomly stacked rounded or irregularly shaped boulder walls.  Since the stones may not have sufficient mass to support retained grade, additional reinforcement can be provided by using exterior grade construction adhesive to “glue” each course together.  These walls are only recommended for shorter applications (less than 3’ tall) with no additional surcharge since they can only achieve structural integrity from wall batter formed during installation and the construction adhesive holding each course together.  Much like stone boulder walls, the advantages of dry laid stone walls include the natural appearance of the material and longevity of the wall when properly installed.  The disadvantages of this wall type include the cost of the construction materials and high level of skill required to properly install.
Cast-In-Place Concrete Walls

Cast-In-Place Concrete Wall
Cast-in-place concrete retaining wall systems maintain structural support through the combined strength of the concrete and steel reinforcing bars (rebar) used to form the wall as well as the depth and design of the wall’s footing. Most concrete walls incorporate a spread footing, which is a wide, flat slab connected to the wall’s vertical stem. The spread footing needs to be at least as deep as frost-depth for the regional climate, but often times even deeper in order to have sufficient stabilized soil cover to prevent over-turning of the wall. These walls can be made decorative with brick or stone veneer, but these facing materials are strictly decorative - they provide no structural support for the wall.  The advantages of cast-in-place concrete walls are their longevity, geometric capabilities (curves, angles, slopes – you name it – can be formed with these walls), and aesthetic options available when cladding with brick, stone, or tile veneer.  The drawbacks of cast-in-place concrete walls are the cost of materials and complexity of design and installation.
As you can see from the all the graphics above, a drainage system comprised of a column of free-draining stone wrapped in filter fabric and perforated drain pipe is recommended for all wall system types in order to relieve hydrostatic pressure.
Finally, required lengths of deadmen and geogrid, depths of footings, and maximum heights walls will vary based on the type of wall, type of soil being retained, additional surcharges being placed on the wall, and other possible factors.  A licensed structural engineer should be consulted on the design of any wall taller than four feet.