Gardening – Versatile hydrangea

Hydrangea are one of the most versatile, easiest and fastest-growing shrubs. They are brilliant for filling a neglected corner, massed planting for low-maintenance gardens, as colourful hedges and for cut flowers. Left to their own devices, they form large, trunkless shrubs, growing via branching stolons that can grow to three metres in height and width in the right conditions, but are easily pruned to suit.

Hydrangea are prone to a range of diseases, including several viruses. For home gardeners though, there is only one disease that has a serious impact, which is powdery mildew, usually appearing late in the season with warm days, cooler nights and high humidity. Pests include aphids, slugs and snails, two spotted mites, passion vine hopper and thrips, but most of these are of little consequence until late in the season.

Aside from the white varieties, which can discolour or burn in full sun, hydrangea produce their best growth and shape in full sun, although they will happily grow in shade. Wind protection is important, particularly in the first year.

In heavier soils, the use of raised beds is recommended to improve drainage. If the soil is not free draining, the root system can rot. In lighter soils, planting on flat ground is preferred so the roots don’t dry out so readily. Hydrangea require a lot of water – insufficient water will cause the leaves to wilt and eventually cause leaf die-off and reduced flower production.

As many gardeners will know, hydrangea flowers change colour in response to soil acidity or alkalinity, and fertiliser inputs.

An acid soil (pH level of 6.0 or less) is essential for bringing out the best blue. Fertilising with aluminium sulphate increases the blue colouration further. Low soil phosphorus levels and a high level of potassium help to produce clear blues. If mauve colours are required, then phosphorus levels can be increased.

Soils which are near neutral or slightly alkaline (pH of 6.0 and above) are suitable for producing pink flowers. High nitrogen inputs, high phosphorus and low potassium levels all assist in maintaining clear pink colours. White varieties are also best grown in these soil conditions.

In soils with pH above 6.0, iron and manganese deficiencies may occur, so foliar feeding with these trace elements is useful, or use a balance foliar feed like seaweed. Fertilising should be carried out just before spring growth. I use sheep pellets, followed by a layer of mulch as the additional organic matter is very good for hydrangea root systems.

The final size of the variety should be considered before planting. Typically, if planting a bed of hydrangea or a hedge, one to two metre wide beds are used, with plants at intervals of approximately one metre in the row. Young plants should have the tips pinched out several times during their growth stage to encourage more branching. Winter pruning can be as light or as heavy as you prefer, with light pruning leading to more but smaller flower spikes and heavier pruning leading to fewer but larger flower spikes.