Ilex crenata Convexa - Japanese Holly

Ilex crenata 'Convexa' is a variety of Japanese holly with relatively slow growth. It features small, non-prickly, glossy green leaves. It is evergreen throughout the year. Its low vigour, dense foliage, and excellent suitability for pruning make it ideal for topiary formation. It is particularly well suited to ball formation, as proposed here. Growing well in semi-shade, or even in the sun, it requires slightly moist, well-drained, neutral to slightly acidic soil. Its good hardiness (-15°C (5°F) approximately) makes it suitable for most regions.

Ilex crenata, the type species, is native to temperate regions of Asia (China, Japan, Russia). It was introduced to Europe in the 19th century.

It bears rigid branches and small, glossy green, convex-shaped leaves resembling the leaves of boxwood and measuring 1 to 3cm (0.4 to 1in) long. Insignificant greenish-white flowers (5mm (0.2in)) appear in May-June. It is a dioecious plant, meaning that a plant is either female or male. In autumn, female plants bear small, shiny black berries, 2 to 6mm (0.1 to 0.2in) that persist all winter. In its native habitat, this bush can exceed 5m (16ft) in height, which is not the case in our gardens.

Left in free form, 'Convexa' will reach 3 to 4m (10 to 13ft) in height and 3m (10ft) in spread in our gardens, but pruned in topiary, its dimensions will be limited according to the desired effect in the garden. Its main quality comes from its dense branches, and its medium green glossy foliage. The slightly dentate (crenate) leaves do not exceed 1 to 2cm (0.4 to 1in). Its growth rate is quite slow, of the order of 15 to 20cm (6 to 8in) per year.

'Convexa' can easily be maintained in a ball shape, by pruning it in spring and a second time in September. With its geometric aspect, this topiary will find its place in Japanese (of which it is a historical component) or contemporary gardens. In summary, this bush is suitable for all gardens where volume and colour take precedence over the individuality of the plant. A particularly interesting visual effect can be obtained in group planting, for example by creating a perfect checkerboard pattern. Juxtaposing 3 or 5 holly balls of different diameters will maximise the volume of these bushes. It will then be enough to let some plants grow for 1 or 2 years, adjusting their shape if necessary, while pruning others to contain their growth. Used as a single specimen, this holly can also be combined with other topiaries, or with structural plants such as columnar conifers with bluish foliage, like Juniperus 'Blue Arrow', to create a contrast in shape and colour. Growing well in slightly acidic soil, many ericaceous plants will make good companions, such as the small Skimmia japonica 'Magic Merlot' with variegated foliage, camellias with their stylised flowers, Japanese maples or the superb Magnolia liliiflora 'Nigra' with its spectacular flowering. It will also be perfect in a beautiful wooden pot (used for orangery plants), or in painted metal, to decorate a terrace or frame a path with several plants.