Some simple advice, even for the math-challenged
It’s not exactly a science but spacing new plantings IS important in garden design.
A 53’ red semi-truck laden with 15 tons of fireworks leaves Cleveland on a 92-degree summer day heading south at 74 mph. At the same time, a 9-car freight train hauling 564 cases of Ben & Jerry’s Ice Cream departs from Burlington, VT, and soon hits a cruising speed of 78 mph. Provided this data, how long would it take a 2 lb. Eastern box turtle to go down a greased, 20’ playground slide at 10:27 am on September 28th?
Don’t ya love a good math problem? Or as Tom and Ray Magliozzi (RIP) from that classic radio show, Car Talk would refer to them, “Puzzlers”. As always, you are wondering where this is heading. (For my fans of diversions, I do NOT want to disappoint). Deciding on plant spacing in our gardens gives us a chance to put our math prowess to the test but let’s face it, most of us just lay out the pots and wing it until it looks right.
How far apart should someone plant azalea that reach a mature width of 6’? How many pachysandra does it take to fill a 200 square foot bed when planted 8” on center (OC)? Determining plant spacing depends on many factors. Ultimate size, species, goals, availability, budget, and more are all things to consider. To keep things simple, I’ll divide this into the different plant categories: trees, shrubs, perennials/grasses, and groundcovers.
Trees
If you plant only a single tree, of course there’s nothing to discuss. But if you have some land and a blank slate, maybe you’re trying to line a long driveway with an allee of shade trees to give it a Tara-like (gone with the wind?) feel. Again, spacing depends on many things but when I’ve encountered this situation as a designer, I would space my circles (trees) about 40’ apart. This is for trees like oak, maple, beech, etc. For more narrow, upright species like dawn redwood, bald cypress, slender silhouette sweetgum, upright ginkgos, I would draw them about 30’ OC. But most of us, myself included, don’t own a ranch with several acres.
The spacing of trees on our property are bound by few rules. The one I do hold by is not to place large trees too close to the foundation. Understanding the rooting patterns of trees is helpful if you have a smaller plot of land. Some trees like silver maple are notorious for having shallow, aggressive roots near the surface. Some 40 years ago we inherited a beautiful dogwood that began its life 15” from the foundation of the garage. It’s still there today and has yet to buckle the nearby patio and walk.
As I write this, I look out and spy several “volunteer” trees in our garden that chose their own place to begin life. Since our yard is mostly beds, I have no problem letting these guys take hold for the next generation. These include sourwood, blackgum, and red oak. I’m now at the age where I’m less concerned with what happens 50 years down the road. Mortality. What are ya gonna do?
Determining spacing for evergreens planted for screening and privacy should also be mentioned. Trees in this group may include dwarf southern magnolia, cedar, large hollies, arborvitae, and hinoki cypress. (Notice I did NOT list Leyland cypress. There are too many other choices to include this troublemaker). Spacing for the above may be around 10-12’ apart, depending on one’s goals, budget, and patience. Slightly off topic of spacing, I should also say that it is advisable to plant a mix of these plants rather than a monoculture. If one croaks down the road, it’s easier to find a replacement if you have choices. Plus, you don’t want to put all your trees (eggs) in one basket in the event some bad cooties come and attack one species.
An idea for plant spacing for a screen. A general idea. Do some research on the mature spread of your desired species and make any adjustments.
Shrubs
I’ve rattled on longer than I needed to about trees so let’s next go to the shrubs. Larger, flowering species like oak leaf hydrangea, forsythia, blueberries, deciduous hollies, and viburnum might be spaced 8’ OC or more. Smaller foundation plants such as yews, smaller hollies, azalea, etc. are typically planted around 4’ OC. If it’s a hedge you’re after and using such plants as abelia, boxwood, or smaller yews, you might consider spacing of 3’ depending on the size of the installed plants and of course, your budget.
For the most part, unless a shrub is meant to be enjoyed as a specimen (single plant with no competition nearby) most are meant to be planted in masses or groupings. The mistake I usually see here and one I’ve mentioned in some of my previous writings, is owners’ or maintenance folks’ compelling desire to force these plants to practice social distancing by pruning down the sides as well as the tops. The result is the ubiquitous “meatball”. Let these plants hold hands people! Pruning shrubs to resemble oversized hors d’ oeuvres is NOT a good look. (Thank goodness for spell check!)
Social distancing of plants is a look to avoid. Put away those shears and allow them to be buddies.
Perennials and grasses
Knowing how to space these requires an understanding of how each species grows. Is it a clumping plant, like most ferns and grasses? Or is it a spreader (rhizomatic) like epimedium or Soloman’s seal? With perennials that spread by self-seeding like columbine or money plant, the spacing won’t matter as much since they’ll go everywhere. For the most part, ornamental grasses are clumping so as you space new ones in the garden, you may want to consider leaving some extra space between them initially as you might later dig and divide these and THEN fill those gaps. One species I do find attractive but it can be a bit of a thug is Cherokee sedge. This grass has no problem both self-seeding and spreading by rhizomes. I should have mentioned in the tree and shrub paragraphs above but a couple other natives you’ll want to keep a check on are paw paw trees, since these love to colonize and sumac for the same reason. Again, should you use these, plant spacing won’t matter since they can be pretty aggressive.
Groundcovers
For larger areas (<80 sq. ft.), calculating the spacing for groundcovers such as ajuga, pachysandra, mondo grass, dwarf plumbago, etc. can be pretty simple thanks to the table below. You simply divide the area by the “Imperial Units / Square Feet” number on the chart to get the number of plants needed. For example: 200 sq. ft. bed with a desired spacing of 10” = 200/1.7 = 117.6 or 118 plugs /plants needed. Personally, I think planting closer that 10” is not only difficult but a waste of money.
For small areas, you might just lay down a yardstick in the bed and eyeball how many plants you need.
So, ready for the answer to my “math” question at the start of all of this? Keep subscribing to find out on a future post……or not.
Till next time,
HAPPY CALCULATING!
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