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Acid test for chameleon hydrangeas
They say:ĚýIf your prize hydrangea won’t turn blue, try using vinegar or coffee grounds to acidify the soil
Hydrangea blooms are touted as a litmus test for soil pH: blue means the soil is quite acidic, pink means the pH is 5.5 or higher. But it’s not that simple. Most hydrangeas have white blooms that go green or pink as the plants age, regardless of the pH. And only two species – French hydrangea (Hydrangea macrophylla) and mountain hydrangea (H. serrata) – change colour depending on their soil.
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The switch is due to a single pigment whose hue depends on its chemical structure. Aluminium ions from the soil bind to the pigment, changing its colour. Having few or no aluminium ions gives pink flowers, having a lot of them yields blue ones, and being somewhere in the middle turns your hydrangea purple. But even when there is plenty of aluminium, the flowers may still be pink or purple. This is where pH comes in: aluminium ions are more freely available and easier for plants to absorb in acidic soils.
“Most hydrangeas have white blooms that change with age, not pH”
If you are desperate for blue blooms, your best bet is to buy a solution of aluminium sulphate. The sulphate will acidify the soil, ensuring the aluminium ions can get into the plant. Watch out though: too much aluminium will poison the hydrangea and potentially its neighbours too. Lime (calcium hydroxide) will decrease the acidity of the soil, so can make your hydrangeas pinker. And always remember that colour changes usually take a growing season or two to show up.
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Chop chop! for longer-lasting blooms
They say:ĚýIf you want more flowers and less plant, cut back your late-flowering perennials in late spring
This one’s a winner. Halving the size of plants such as sedum, rudbeckia, echinacea and helenium in late spring is known as the Chelsea chop, after the Royal Horticultural Society’s annual Chelsea Flower Show in London. The aim is to get more flowers in one season. It works because it stops apical dominance – when hormones produced by the growing tips of a plant stop side-shoots forming. Cut the growing tips and you will get bushier plants with more opportunities for flower buds.
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A daff idea: tying up spring leaves
They say:ĚýTidy up your daffodils by tying up their large leaves after they flower
Leaving daffodil leaves in place after their big show is over is a must: they will carry on photosynthesising and storing energy in the bulb. Tying them in knots may look tidier but it’s a bad idea. It damages the leaves and hampers photosynthesis. The result may be an inferior display the following year.
Instead, just cut off fading flowers, which stops the plant from putting too much energy into producing seed.
This article appeared in print under the headline “Growing skills”
