Hydrangea macrophylla 'Holstein'

A deciduous shrub growing to 1.5m tall and 1.5m wide dependant on pruning.

Large rounded heads of many blue flowers are produced from early summer through to autumn. This particular hydrangea takes on more antique tones as the flowers mature in autumn, turning reddish-purple. Dark green foliage is glossy with pointed tips on the leaves.

Grow in moist but well-drained soil in full sun or partial shade. Can endure harsh winter cold and is drought resistant. Hydrangea thrive in shade, however, they can also be grown in full sun but the flowers of white and blue cultivars are prone to burning.

Always choose healthy, well-grown, plants and plant after autumn rains. Planting then when the soil is moist and warm allows shrubs to become established before winter. This enables them to withstand dry periods during the following summer. Planting success is often improved on clay soils by adding extra topsoil and forming raised beds. Incorporate coarse sand, bark, peat, compost or other organic material to improve soil structure. Before planting, ensure that the root-ball is saturated and remove planter bag pot or wrapping with the minimum of root disturbance. Trim any broken roots and plant at the same level as in the container. Dig a hole twice the diameter of the root ball and firm in and water once planted. Plant with some general slow-release fertiliser and then every spring apply an organic-based fertiliser such as blood and bone at a handful per square metre as new growth begins. Planting too closely leads to spindly growth, poor flowering and eventual decline. In a well-planned border, the plants in flower should just touch each other to create a full effect without overcrowding. Plant approximately 1.5m apart. 

Apply an organic mulch annually to help suppress weeds and conserve moisture. Organic materials such as sawdust and bark contribute to soil structure as they decompose. Keep mulching material away from plant stems. The first summer and autumn after planting is critical for young plants; water thoroughly during dry periods. Except on very fertile soils, an annual light dressing (50g/m2) of general-purpose fertiliser in spring is beneficial. Sprinkle evenly and work into the top 2 to 3cm of the soil, taking care not to damage surface roots. Prune off any old flowers after flowering. Delay pruning until the danger of frost has passed as the spent flowers and leaves protect buds lower down from damage. Cut out all canes which have flowered as well as all thin crowded and damaged shoots right to the base. Leave unpruned shoots which have not flowered as these will flower next season. Pests and diseases can have serious debilitating effects on young plants; check regularly.

Flower colour is affected by soil acidity (pH); acid soils producing blue and alkaline soils pink flowers. White flowers are unaffected. Most Auckland soils are acidic and therefore blue flowers tend to predominate. The blue colour may be intensified by watering plants with a solution of 44gm of aluminium sulphate in 9 litres of water in early spring. Use the same solution for container-grown plants. Coastal gardens with deposits of shell in the soil are usually alkaline producing pink flowers; the pink colour is intensified with applications of lime especially hydrated lime, at 100/m² in autumn each year. Heavy applications of lime up to 500g/m² can be applied to change blues to pinks. Such efforts usually result in intermediate purplish tones rather than a clear pink. Grow plants in containers using an alkaline potting mix for pinks and an acid mix for blues intensifying the colour with applications of the chemicals described above.

Attracts bees and pollinators.

Most of the time hydrangeas are trouble-free, however, occasionally they can suffer from grey mould, slugs, powdery mildew, rust, ringspot and virus leaf spots. If grey mould is a problem, keep humidity low and remove the plant or affected plant parts. For powdery mildew, spray with baking soda solution. If infected with rust, remove fallen leaves or use more resistant cultivars. Virus affected plants should be removed and burnt.

Trial Garden