Under the Spruce Tree
by Rob Sproule
In the gardening world some questions are as variable as the current trends but others, like “what can I grow under my spruce tree”, are a constant. Every few days someone asks or e-mails me this one.
Why won’t anything grow there?
As any doctor will tell you, proper treatment requires proper diagnosis. There are actually 3 reasons why all your grass dies under large spruce (and pine, etc) trees.
The first problem is acidic soil. Spruce and pine needles have a pH of about 4.5-5, over a full point lower than the optimum 6-6.5 range of most plants. To us 1-2 pH points may not seem like much, but to flora it’s a deal-breaker.
The second problem is moisture. A 50′ spruce tree requires a staggering amount of water. While the myth is that spruce roots burrow deep enough to pull moisture from the depths, the truth is that most collecting roots are within 30 cm of the surface.
Spruce roots are very powerful, as anyone with a basement next to them can tell you. Any moisture within a spruce’s drip line gets greedily sucked up before other plants can use it. Your under-tree lawn died as much from thirst as it did from acidity.
Last and least, the third reason why so little will grow is nitrogen deficiency. Between the tree sucking up nutrients and microbes, who are breaking down the fallen needles, taking what’s left, there’s precious little leftover. Unlike the former, more challenging problems, you can solve this one with a little extra fertilizer (look for bags with a high first number).
Solutions
All is not lost. There are a range of acid-loving plants that will thrive around the tree. As a rule of thumb, the farther out from the trunk you get, the easier it will be for these varieties to thrive. Especially if you add an evergreen and acid-loving plants food, like Miracle Gro.
Notable acid-loving shrubs and perennials include: rhododendrons, blueberries, bergenias, hostas, lungwort, lily-of-the-valley, and any juniper or other conifer,
Hydrangea will thrive just outside the drip line, and the acidity will actually turn their flowers bluer.
When planting around a spruce, even outside the drip line, make sure to provide supplemental water. Even if it’s not a shrub you would normally water remember that it’s fighting a losing battle with much larger spruce roots.
Scope out where the large roots are first and flag them; roots and shovels don’t play well together. Plant in areas farthest from the roots and add lots of compost and other nutrient rich goodies to give the newbies a good head start.
If you don’t want to deal with the parched, acidic soil, try pots of colour. Impatiens, begonias, and a host of other shade lovers can nestle up against the trunk without fear of dying. Needles will fall into it, but not enough to kill the plants before winter does.
I spoke to gardening author and Salisbury’s Nursery manager Ali Beck, and she suggested submerging pots in the ground and filling them with annuals. You would get the same advantage over acidic soil and water/ nitrogen depletion as you do with the above ground pots, but the illusion would be that – somehow- you’ve made Impatiens and Begonias thrive under a spruce tree! Don’t be surprised if passers-by knock on your door to ask how.
