soil pH

The 'Dirt' on Hydrangeas

The 'Dirt' on Hydrangeas

By Jim Hole

Fact or Fiction?
You can change the colour of your hydrangea flowers by making the soil acidic.’

Hydrangeas are some of our most spectacular flowering shrubs. And while there are over a dozen great varieties that grow beautifully on the prairies, there are those among us who just can’t resist the challenge of changing a pink flowered hydrangea to one that flowers blue or vice versa. Today there are over a dozen varieties that we can grow here successfully.

But can one really change hydrangea flower colour? The answer is yes…well, sort of.

Changing the colour of hydrangea flowers starts with understanding a bit about soil chemistry and then choosing the right varieties. In our greenhouses, I was the guy who was in charge of adding the correct ingredients, in the right proportions to the soil half of the hydrangeas would flower blue while other half would flower pink. But I’ll be the first to admit that I didn’t always get it right. More often than I care to admit, I ended up with what are known as ‘blurple’ hydrangeas – mostly blue but with enough red blended in to give the hydrangea flowers a purpley tone. Now, I thought the blurples were rather attractive but, apparently, that sentiment wasn’t shared by everyone!

So how does one get a red, blue or even a blurple hydrangea for that matter? It all begins with choosing hydrangeas that have the capacity to change colour. The vast majority of the hydrangeas that we grow here are incapable of changing colour regardless of what you do. For example, white hydrangeas will remain white regardless of what treatments you provide.

If you have responsive hydrangeas then the next step is to raise or lower the soil pH above a threshold level depending on whether you want blue flowers or pink flowers. If you want a pink hydrangea, the soil must be fairly alkaline (higher pH) but if you want a blue hydrangea the soil must be rather acidic (lower pH).

Diving into soil chemistry just a bit deeper, acidic soils make aluminum (a naturally occurring soil element) more soluble and more readily absorbed by plant roots whereas alkaline soils make aluminum less soluble and thus more difficult for roots to absorb. At the cellular level the aluminum alters the pigments in the hydrangea blooms and, voila, the flower colours change. But the caveat here is that if you don’t tweak the soil correctly, you’ll end up with my blurple colour.

Keep in mind that only a select few hydrangeas are responsive to manipulation of soil pH. In you want to experiment, a variety called Bloomstruck is one variety to have some fun with.

Remember too that once an existing flower is already pink or blue, it won’t change colour. Tweaking soil acidity will only affect the coloration of flowers that have yet to develop.

Also, in the garden, pH manipulation can be very difficult particularly if you have a clay-loam soil with lots of lime in it. Acidifying this type of soil is nearly impossible so just be satisfied with growing a healthy, floriferous hydrangea and enjoy whatever colour you get!

The florist type hydrangeas (hydrangea macrophylla) are the best hydrangeas for having some fun with flower colour transformation. They are only marginally hardy outside but are great for playing around with outside in containers during the summer. They love morning sun but hate intense afternoon sun so place them in a spot where they won’t suffer from sunburn.

Remember that even if you change soil pH beyond certain threshold levels, you won’t get a rainbow colours from your hydrangea flowers. Pink and blue are your only two choices…and blurple, if you weren’t paying attention in your soil chemistry class.

Some great Hole’s Hydrangeas to grow:

  • Annabelle, Bloomstruck, Bobo, Incrediball, Limelight

The pHacts on pH

The pHacts on pH

By Jim Hole

Whenever a discussion about the effects of soil pH on plants comes up, I'm willing to bet that many gardeners are a little mystified by the issue. Almost everyone knows that pH has some kind of impact on plants, but by and large it seems that people are still unclear on the specifics.

Soil pH really isn't that complicated. It's simply a number that represents the degree of acidity or alkalinity (also called basicity) of the soil. pH scores run from 0 (very acidic) to 14 (very alkaline or basic), with 7 being neutral. (Distilled water has a pH of 7). In the real world, soil pH values are never as low as 0 or as high as 14.

My Cup of Tea

To visualize how pH works, I like to use the tea analogy. Have you ever noticed how quickly and easily sugar dissolves in hot tea versus how slowly it dissolves in ice tea? Soil with a high pH is like cold tea; soil with low pH is like hot tea.

Most, but not all, nutrients are affected by pH as follows: at a very high pH, insufficient nutrients are dissolved for the plant roots to absorb, and the plants suffer. In other words, the tea isn't sweet enough. At a very low pH, many nutrients and some toxic metals are too soluble in the soil—so soluble, in fact, that they may be a little bit too available to the plants, resulting in injury. In this case, the tea is too sweet.

Ideally, your cup of tea should be neither too sweet nor too bland—you want a pH that's "just right." For most garden plants, a soil pH between 6.2-6.8 is ideal. (Some plants, of course, do like the extremes: acidic for azaleas, rhododendrons, and blueberries versus basic for gypsophila, Russian olive, etc.).

When soil pH falls a little outside the desired range for your plants, it can be adjusted. Sulphur, sphagnum peat, and aluminum sulphate lower soil pH, whereas limestone will raise it. Often, garden soils are in the correct pH range, but the only way to be certain is to have them tested. Soil pH test kits can be found at most garden centres, though they vary in quality. I've found the cheap $10 pH meters to be essentially useless. Reasonably good-quality pH meters start at around $50. A mid-range alternative is one of the kits that involve pouring powders into test tubes and using colour as an indicator of pH value. These are, generally speaking, quite good. The kits aren't as precise as a quality meter, but they give a reasonably accurate reading. Soil pH isn't really that complex when you take a closer look. A little basic knowledge will help
you pass the acid test.


pH is an abbreviation derived from the French phrase "pouvoir hydrogène", or the power of hydrogen. It refers to the concentration of hydrogen ions in a solution. pH values are on a negative logarithmic scale. What that means is that the difference between pH 5 and pH 6 is larger than it first appears; 5 is 100 times more acidic than 6; and, going further, 4, is 1000 times more acidic than 6. So if your plants happen to require a soil pH of 6, 5 definitely isn't close enough.

Soil pH has a tremendous effect on soil microorganisms. Some soil microorganisms work hard to convert compost , manure, and other organic matter into usable nutrients for your plants. Such organisms can't survive in extremely acidic soil, leaving your plants with few nutrients to absorb.