After such a horribly hot summer, many of my Hydrangea bushes are looking very much the worse for wear. The flower heads were severely burned in the 45-degree day of 18 January. I usually don't prune my Hydrangea until winter but this year I am thinking of pruning this month, to tidy them up. I usually like to leave on the old flower heads to enjoy the antique shades of purple, green, russet and bronze as they age but this year the predominant colour is brown - not a good look! To prune, I remove spindly stems and cut the stems back to the lowest pair of plump buds. A couple of very gnarled old stems can be removed at ground level each year on older shrubs. Most of my shrubs are the old-fashioned 'hortensia' (macrophylla) types: tall, robust plants with thick stems, many of which I have grown from cuttings given to me over the years.
However, I have had a few smaller-growing types with much thinner stems, and I have usually pruned these in July with my other bigger ones. They have never flowered very profusely. Last year I attended an interesting presentation on Hydrangea from Mark Adamson from Heritage Plants and learned that these smaller-growing ones should be pruned in February, cutting the whole plant back by half, otherwise they don't have sufficient time to regrow flowering wood for the following late spring blooming. This was a complete revelation to me, so I will be definitely always pruning these smaller ones that way now. These include Hydrangea serrata cultivars such as 'Blue Deckle' and 'Grayswood', which I have always assumed are not great doers in Sydney. I am hoping the new pruning method will bring me more flowers next spring.
In recent times there have been some new releases of smaller-growing Hydrangea macrophylla, including the 'Endless Summer' collection, which has mophead and lacecap forms in white and pink/blue colours in various shades. I recently acquired a lacecap one (named 'Twist-n-Shout') and it has flowered very well in a pot - it can be pruned back several times to get a new flush of blooms. Another recent acquisition of these more compact ones was Hydrangea macrophylla 'You & Me', which grows about 80 cm tall and has lovely double blue flowers, also growing in a pot. These have now aged to a gorgeous greeny-pink hue (as it was in a much more protected position than the ones I have in the garden so wasn't burned), which I am enjoying. I plan to cut these back in late February to make sure they flower well next year. The tall, old-fashioned lacecap varieties are pruned more lightly in February too, as the flower heads don't ever age attractively like the mopheads. Cut them back just above the topmost pair of buds. The shrubs can be fed after pruning to encourage the new growth and fed in late winter as well.
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. 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!
Hydrangeas planted in the ground should be given good, compost-enriched soil and be kept well mulched. Hydrated cocopeat can be added to the planting soil to help retain moisture. 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. I obviously need to move some of my specimens!
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