Plump Hydrangea bushes, with their bold blue or pink mopheads, are wonderful plants for our Sydney climate, blooming over an extended period from late spring through summer, and being attractive even as their flowers age in autumn. The fact that they enjoy shady parts of the garden is an added bonus. They provide excellent cut flower material as well (especially for Christmas). Many of the more compact types grow very well in large pots, which can be moved around to show off the plants when they are in full bloom.
The status of Hydrangea macrophylla has risen in recent years, with many fancy cultivars now available. The big domed mopheads (often known as 'hortensias') are the most commonly seen. The pretty lacecap varieties, with an outer ring of sterile florets surrounding a clustered centre of tiny fertile flowers, are very popular with gardeners at the moment. Reliable cultivars include 'Libelle' (ht 1.5m) and 'Lanarth White' (1.5m), both with white florets. Coloured ones are also available. There are forms which have double flowers (such as the stunning white 'Fuji Waterfall'), which are intriguing and attractive. Some gardeners report that their lacecaps self-seed - it's worth potting some of these up to see what might eventuate! I have a variegated-leaf lacecap (possibly H. 'Maculata') and an unusual Hydrangea macrophylla cultivar named 'Ayesha', which is a very strong, tough shrub, growing almost 3m tall. Each flower has a cupped, waxy form, a bit like a lilac - our local florist refers to it as 'popcorn Hydrangea', and each flower-head does actually look like a cup of blue (or pink!) popcorn! Hydrangea heights vary from 1-3 m.
Hydrangea flowers vary from pink to blue depending on the pH of the soil - an acid soil results in blue flowers, whilst an alkaline soil gives pink blooms. The reason for this is that there is a pigment in their flowers that is blue when it binds with aluminium and red when it doesn't. Flowers can take up more aluminium from the soil under acidic conditions (pH of 5 or less), resulting in blue flowers; when the soil is neutral or alkaline, pinker blooms result . I have a pink mophead called 'Tosca' which is supposed to stay always pink because it doesn't take aluminium up from the soil at all, but even it has been known to turn blue in some gardens. The compact 'Pia Mia' (ht to 1 m) is another that is said to stay pink no matter what. White forms are stable, though the 'eye' of the flowers may be tinged blue or pink depending on the soil. There are red ones available - I'm not sure how these fare in different soils. The colour intensity of a particular Hydrangea cultivar - whether a pale or strong colour - seems not to vary with pH, according to some growers. Various substances are available from nurseries to manipulate the colours - such as alum for blue flowers and lime for pink ones. The presence of phosphorous in the soil is also apparently a factor, so adding superphosphate will promote pinker blooms: for blue flowers you can use a plant food designed for native plants, which is low in phosphorous. These substances may take about a year to take effect, however! Another interesting observation has been that wood ash added to the soil will result in pink hydrangeas, as it has an alkaline effect, which may act quite quickly on the flowers.
Hydrangea can look hideous and straggly if they are neglected and unpruned, but as long as they are given some water and a bit of attention, they will reward you many times over. The best time to plant is autumn, winter or very early spring - not in the heat of summer! Their need for water can be reduced somewhat by planting them from the outset in good, compost-enriched soil and keeping them well mulched. Hydrated cocopeat can be added to the planting soil to help retain moisture. Anti-desiccant products can be tried on the leaves to help the plants cope with hot weather especially in their early years. Once they are established, they need less watering. It is best not to plant them too near to large trees with greedy roots - they grow best in the shade cast by buildings where there is no root competition. They don't mind having some morning sun - and in fact will flower better for it; it is the hot afternoon sun that will singe their leaves and burn their blooms. The blue- and red-flowered types seem to cope with more sun than the white-flowered ones.
They seem to have few pests or diseases, though fungal problems can occur in the leaves during the humidity of summer. The worst leaves can be picked off - don't compost them, however. These, and any affected leaves that fall on the ground should be put them in your green waste council bin. A spray of milk and water may help combat the mildew. Sometimes leaves may seem distorted, but this is due to sudden spells of heat in spring. Red spider mite can be a problem - insect predators can be used to control them, if required. For the tall, strong-stemmed, old-fashioned mopheads, pruning can be done either in February or July, depending on whether you want to leave the ageing flower-heads, which can be very attractive as they turn to interesting colours of antique pinks, purple, bronze, russet and green.
To prune, remove spindly stems and cut the stems that flowered in the previous summer back to the first plump pair of buds under the old flower head. A couple of very gnarled old stems can be removed right at ground level each year on older shrubs. Leave unflowered stems, as they will bloom next year, unless these are very tall and out of proportion to the rest of the shrub. For the modern, thin-stemmed, lower-growing cultivars (such as the 'Endless Summer' and 'You & Me' ranges, which are wonderful for pots), it is best to prune in February, cutting the whole plant back by about half - otherwise you will get few flowers. The tall, old-fashioned lacecap varieties are pruned in February too, as the flower-heads don't age attractively like the mopheads. Cut them back to the second pair of leaves below the old flower heads. Feed well after any pruning done in February. All plants can also be fed in late winter. Osmocote can give good results.
Hardwood cuttings can be taken in winter - they will take a while to strike but will eventually form new plants. It is also apparently possible to take tip cuttings in summer.
To pick Hydrangea for a vase, ensure that the flower heads are fully open otherwise they will not last well; cut the stems in the early morning and keep immersed in a bucket of water, up to the flower heads, for several hours before arranging in your vase. I was told as a child to bash the end of the stems with a hammer so we still do that - supposedly this helps the stem take in more water. Remove most of the leaves when placing them in your vase and make sure there is plenty of water in the vase. Add a flower preservative sachet if you have one and change the water regularly. Stems picked in autumn can be dried to provide a long-lasting floral display. Strip off the leaves and put the stems into a vase of water until the flowers become like paper.