I really love hydrangeas and last year planted a selection of them in my garden. But as the plants have started to come back into leaf this month, I’ve noticed much to my disappointment that the foliage is slightly yellowed and the growth is poor. Any tips on what’s wrong with them? Jenny M, Co Kilkenny
These large, deciduous, woody plants are stalwarts of the summer and autumn garden and are prized for giving valuable structure as well as plenty of colour in the shape of their big, long-lasting flowers. Several different ornamental species are commonly grown in Irish gardens. They include the mophead and lace-cap hydrangeas (Hydrangea macrophylla) with large spherical flower heads; the paniculate hydrangea (Hydrangea paniculata), whose flowers are more cone-shaped; the large, vigorous Hydrangea arborescens of which Hydrangea ‘Annabelle’ is probably the best known variety; the ever-elegant, velvety-leaved Hydrangea aspera ‘Villosa Group’, and the handsome oak-leaf hydrangea, H. quercifolia. A few species are also grown as decorative wall shrubs or climbers including Hydrangea petiolaris and Hydrangea seemannii.
Hydrangeas are thirsty, greedy plants that need a humus-rich, deep, fertile, moisture-retentive soil as well as protection from hot sunshine and desiccating winds to thrive. They also dislike a very alkaline, lime-rich soil, which causes chlorosis or yellowing of the leaves and stunted growth. Given that your garden is located in Kilkenny where free-draining, lime-rich soils are common, I suspect that this is the problem.
Unlike the low pH of a very acidic soil where the issue can be solved relatively simply with the addition of garden lime, it’s more difficult to effectively lower soil pH. Amendments such as sulphur, aluminium sulphate and sulphate of iron can be applied but can be expensive, difficult to use well, and are ineffective on a large scale. Once upon a time gardeners would have added copious quantities of peat, but this is no longer considered environmentally acceptable.
RM Block
Instead, I’d suggest using generous organic mulches of well-rotted manure, leaf mould, home-made garden compost, pine needles or a commercial product such as Gee-Up (geeup.ie) or Enrich (enrich.ie) around the base of the plants every year, spreading it in a 5cm thick layer in spring and then again in autumn. To speed things up, you could also dig up the plants this autumn after leaf fall and incorporate the same well-rotted organic mulch (a mixture is fine) into the planting holes before replanting them.
[ How do I prune a hydrangea?Opens in new window ]
Another alternative is to create some large, raised beds using imported, high-quality, screened topsoil with a normal pH. But bear in mind that this will be costly and laborious and you’ll need to keep the soil regularly mulched as above to prevent any lime in the groundwater from gradually raising the pH over time. Faced with this problem, some gardeners instead grow their hydrangeas in very large pots or tubs, but this is challenging in terms of keeping these thirsty plants properly irrigated and well fed, while they’re also likely to quickly outgrow them.
