Hydrangeas and the science of color | Sowin 'n' the trowel

Every year it seems a different plant will come into its own and shine a little brighter in the garden.

You might get a bumper crop of beans or broccoli or Brussels sprouts.

Or maybe the day lilies are particularly plentiful, the roses bigger and more fragrant or the lavatera actually ended up to be deer-proof.

I’ll leave it to you to decide which plant has been a stand-out in your garden, but as I’ve traveled the island this summer, the plant that’s stood out for me is the hydrangea, especially the mop head (Hydrangea macrophylla), with its ginormous round inflorescences, and the lacecap (Hydrangea macrophylla normalis), which has a gazillion tiny fertile flowers in the center of a bloom ringed with showy infertile flowers.

Ginormous and gazillion may not be very scientific — or good English — but I think they hit the nail on the head when describing these stunning flowers that can vary in color from deep pink to purple and even deep blue.

Though some varieties are guaranteed to stay within a color range, they have been genetically altered to do so.

The majority of H. macrophylla, on the other hand, can and will change color over time.

It all depends on the plant’s access to aluminum in the soil.

If more aluminum is taken up, the flowers will be blue; if less, the flowers will be pink.

It’s not the nursery you should rail at when three years down the road your hydrangea doesn’t look like the one you brought home. It’s just Mother Nature’s chemistry set in action.

If you want your hydrangea to be pink and it isn’t, add dolomite lime to the soil several times a year to raise the pH.

Aluminum absorption is inhibited by a higher pH.

Don’t go overboard with the lime, however, because if the alkalinity of the soil is too high, your hydrangea may experience an iron deficiency. Use a high phosphorus fertilizer as well to bolster the effects of the lime.

To get a blue flower, lower the pH with coffee grounds or other organic matter so the plant can more easily absorb aluminum and add aluminum sulfate to the soil. Again, your choice of fertilizer can help this process along.

Avoid high phosphorus fertilizers and bonemeal, and instead ramp up the potassium.

And don’t plant your hydrangea near anything made of concrete.

Though it sounds simple in theory, you as a gardener have only limited power to affect this color change.

If you’ve got naturally occurring aluminum in your soil, you’re going to forever be fighting it to get pink flowers.

If in that case you’re still determined to go pink, then try growing your hydrangea in a pot in a soilless potting mix where you can control all aspects of your plant’s nutrient uptake.

Just don’t bother to try this science experiment on naturally white hydrangeas, such as Hydrangea arborescens or Hydrangea quercifolia (oak leaf hydrangeas.)

It won’t work.

What’s more, you’ll come out of it feeling like you’ve been attempting to build a house of cards in a wind tunnel.

Sort of like trying to grow perfect tomatoes in Western Washington.


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