Gardeners frequently wonder how to force their hydrangeas to produce the colour flower they desire – some crave pink flowers while others favour blue. Well sit and wonder no more – here’s how you do it.
Hydrangea colour basics
Contrary to popular belief, pH is not the controlling factor for hydrangea colour. Instead, it is the uptake of aluminium that affects the flower colour. If a hydrangea takes up aluminium the flowers turn blue, if it doesn’t take up sufficient aluminium, the flowers turn pink. Now pH can affect the ability of a plant to uptake aluminium (low pH improves soil availability of aluminium while high pH ties up the aluminium in hydroxy species – other factors can also cause aluminium to form other complexes too) but if the soil contains very little aluminium, no amount of acidification will turn flowers blue.
How to get blue hydrangea flowers
It is easier to turn hydrangea flowers blue than it is to turn them pink. As blue hydrangea flowers require aluminium, you can force hydrangeas to produce blue flowers by adding aluminium to the soil. Now before you head down to your nearest home improvement centre, you don’t want to add a lump of aluminium metal – it would take a long time for that aluminium to become available to your plant. Instead, try aluminium sulfate applied at least once a year (but likely more frequently). This is a form of aluminium that is available to plants and it will acidify the soil which, as noted above, helps improve aluminium availability in soil. If there is plenty of aluminium in the soil but the pH is high (the soil is alkaline) adding sulfur or iron sulfate will also acidify your soil and improve aluminium uptake. Ideally, soil pH for a blue hydrangea should be 5.2-5.5.
Your choice of fertiliser can help you to maintain blue flowers. A low phosphorus and high potassium fertiliser can promote aluminium uptake. It’s also worth checking the pH of your water – if it’s too high, it will raise your soil pH over time. Try using rain water or acidify your water slightly.
If your soil is very low in aluminium and/or very alkaline, you may find it difficult to keep your hydrangea flowers blue. If so, you could consider growing a plant in a pot filled with potting mix designed specifically for growing blue hydrangeas.
Growing pink hydrangeas
As mentioned above, pink hydrangea flowers are harder to obtain if your soil conditions are not appropriate. You cannot remove unnecessary aluminium, so if your soil has too much aluminium your only choice is to prevent your hydrangea from taking it up. Raising the pH using lime can help to some extent but keep the pH at or below 6.2 or your plant is likely to suffer deficiencies of iron and other nutrients. The use of a high phosphorus fertiliser can also help prevent aluminium uptake by a hydrangea. If your soil is high in aluminium and/or the pH is naturally low, you may have better luck growing your hydrangea in a pot filled with a potting mix designed for growing pink hydrangeas.