Geranium macrorrhizum
Geranium macrorrhizum is the cranesbill that solves dry shade. It builds a dense mat of aromatic lobed leaves that release a resinous scent when brushed, and in most New Zealand climates it keeps that cover through winter, thinning only in the coldest districts. Magenta-pink flowers on short stems appear from October through December, sitting just above the foliage rather than over it, and the leaves take red tints through winter.
This is a groundcover first and a flowering plant second. Use it under deciduous trees, along a shaded fence line, on a bank, or as an edging to a shrub border where weeds keep pushing through. It knits together within two seasons and shades out annual weeds. Awkward strips between a path and a wall are where it earns its keep.
Sun or deep shade both work, and so does dry soil once plants are established. It travels by thick surface rhizomes, so it does spread, though the growth is shallow and easy to pull back where it goes too far. It performs throughout New Zealand. Plant 40 cm apart for cover in two seasons. Shear the mat over after flowering if it looks tired.
Foliage sits at 0.3 to 0.4 m, spreading steadily to a metre or more across.
Trimming: Shear the entire mat back by about half after flowering in January if the foliage has gone shabby, and fresh leaves will fill in within weeks. Trim back any rhizomes that have crept past their allotted space with a spade cut through the edge of the mat. No other cutting is needed.
Deadheading: Not required. The flowers are small and drop cleanly, and the plant is grown mainly for foliage cover, so there is nothing to be gained by picking over it.
Fertilising: An application of balanced slow-release fertiliser in early spring keeps the foliage dense, though the plant survives perfectly well without any feeding. A compost mulch at planting time helps it knit together faster in poor or new-build soils.
Watering: Water through the first summer while the mat is closing up. Once established, this cranesbill is among the few groundcovers that genuinely cope with dry shade under trees, and regular watering becomes unnecessary except in extended drought.
Pest Control: Very few problems. Slugs and snails may graze new growth in spring without doing lasting harm, and the aromatic foliage deters most browsing. No routine control is needed.
Disease Management: Rust occasionally appears on older leaves in humid weather and is dealt with by shearing the mat over. Root rot only occurs in waterlogged ground. Otherwise this is a genuinely trouble-free plant in New Zealand conditions.
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