(Originally posted: June 1, 2021)
It’s been a flashy time of year in my neck-of-the-woods. For weeks, the floral fireworks ignited by our trees and shrubs have been creating quite the show in a colourful progression... Witchhazels! Maples! Forsythias! Cherries! Magnolias! Serviceberries! Flowering Pears!
…and…
Rhododendrons!
Floral fireworks of spring.
It is this time of year, when I longingly look at the blooms of the Rhodos. My feet then get ichy to ‘put the pedal to the metal’ and zip out to the garden centre to get a few (it’s best to buy in 3s, right!?) of my own.
Now I should say that kids are amazing at distracting one from the task at hand, so impulse purchases such as this have greatly decreased in recent years. This also affords me the time to let common sense prevail and I am then able to remind myself that these plants are simply not the right fit for my yard. Why? For me, the root problem is soil pH.
How I long for one of these in my garden! Alas it is not meant to be!
Let’s backtrack.
All plants have their preferences when it comes to growing conditions. Some like sun, some prefer shade; some like ample moisture, while others thrive in dry conditions. There are some plants that prefer humus rich soils while others perform poorly in these conditions. Generally, we factor-in these growing preferences when selecting plants for our gardens. To a certain extent, we can control these conditions- by situating plants in various areas of our yard that have the lighting/moisture conditions suitable to the plant at hand.
Two plants with very different preferences in growing conditions. Butterfly weed (Asclepias tuberosa) on the left that prefers sunny, dry conditions on sandy soil while ‘All Gold’ Japanese forest grass (Hakonachloa macra ‘All Gold’) shown on the right, prefers partial shade and deep rich soil.
There is one growing preference that is VERY easy to overlook, or even forget about entirely, but it can often be the key to a particular plant’s success or failure in a garden. Furthermore, it is a soil attribute that we have very little control over and essentially no power to permanently change. That attribute is soil pH.
Don’t worry. I am not going to get into any in depth chemistry lessons here. Essentially pH is a measurement (from 0-14) of how acidic or basic a solution is. A pH of 7 is neutral; anything greater than 7 is alkaline/basic (such as baking soda which has a pH of 8); a pH of less than 7 is acidic (such as lemon juice with a pH between 2-3). The optimal soil pH range for most (but not all) plants is between 6-7. Nutrient availability to plants is linked to soil pH and it is in this ‘optimal pH range’ that the nutrients necessary for plant growth can be found in their soluble forms (in ideal amounts) and therefore available for the plants to take in. Outside of this range, various nutrients can become ‘locked up’ in insoluble forms that the plants cannot access and nutrient deficiencies can result. On the flip-side, at certain pH levels, some elements can become ‘too available’ to plants- to the point where they become phytotoxic.
So what does all this mean for ‘us gardeners’? KNOW YOUR pH! A simple test kit can be purchased at your local garden centre. Alternatively, you can send soil samples off for testing at a laboratory. The reports from such tests are really useful as they generally include measurements of soil nutrient and organic matter levels. This can indicate whether or not amendments are required, and what amendments would be needed to optimize your soil for plant growth.
So- let’s go back to my plant lust example: the Rhododendron. These beauties are considered acid-loving plants- a special club whose membership also includes: blueberries (Vaccinium), mountain laurels (Kalmia latifolia), Japanese pieris (Pieris japonica), pin oak (Quercus palustris) and the list goes on. These plants prefer to grow in soils ranging in pH from 4.5 to 6. If soil pH is too high, these plants will exhibit chlorosis (a sign of iron and/or manganese deficiency- two elements that become more ‘locked-up’ at higher pH levels).
A hydrangea with chlorotic leaves
…but I have slightly basic soil. What are my options?
Generally, soil pH is determined by what lies beneath- the parent material. Since pH is essentially guided by the foundation from which the soil was formed, it means that pH is not exactly an attribute that one can permanently change by sprinkling a few pellets or powder on the ground! The overlying pH tendency of the soil will always prevail. Temporary changes to pH can be accomplished through the routine addition of amendments. For example elemental sulfur or aluminum sulfate can be used to acidify the soil while lime can be used to make a soil more alkaline, but this will become a yearly task to add to the gardening ‘to do’ list.
When it comes to soil, I am not really interested in trying to fight my conditions. So in this case I fall back on the mantra that we should all chant as we walk through the temptations that line the aisles of the garden centres:
”Right plant, right place”.
