Plant problems

Why Are My Hierochloe Leaves Curling?

Q: What does hierochloe odorata smell like?

Hierochloe odorata (holy grass, sweetgrass, vanilla grass) has one of the most distinctive and pleasant scents of any UK grass; the fresh leaves have a mild, sweet, hay-like quality, but the scent intensifies dramatically when the leaves are dried, cut, or crushed, releasing a warm, sweet, vanilla-like aroma that is produced by the chemical compound coumarin (the same compound that gives dried hay its characteristic sweet smell and that is also found in tonka beans and sweet woodruff). The scent of dried hierochloe odorata is a warm, sweet, slightly herbaceous vanilla with hay-like undertones; it is immediately recognisable and quite different from the mild scent of fresh-cut lawn grass. In UK and European tradition, hierochloe odorata was strewn on church floors and steps on saints' days and religious festivals (giving it the common names 'holy grass' and 'Mary's grass' in several European languages), as the scent was released when the leaves were walked upon; in Slavic and Baltic countries, this tradition persisted into the nineteenth century and the grass is still sold as an aromatic herb in some markets. In North America, hierochloe odorata (sweetgrass) is a sacred ceremonial plant used by many Indigenous peoples for smudging, basket weaving, and ceremonial purposes; the dried braided sweetgrass bundles sold in artisan and natural products markets in North America are typically made from hierochloe odorata. The scent is most concentrated in the dried leaves and is released gradually over time from dried braids or bundles of the grass.

Q: How do I grow hierochloe odorata in the UK?

Hierochloe odorata grows best in a partially shaded to lightly shaded position in moderately moist to moist, moderately fertile, neutral to slightly acid soil; it thrives in the kind of cool, moist, lightly shaded conditions found at the edge of a light tree canopy, in a moist, north-facing or east-facing border, in a bog garden with adequate moisture, or alongside a small pond or stream. Full sun is tolerated in consistently moist soil but the plant looks less fresh and shows more leaf curl in a hot UK summer in a dry, sunny position than in a cooler, moister, partially shaded one. Plant from spring to autumn; hierochloe spreads by rhizomes and can colonise a significant area of moist, shaded soil over several years, forming a dense, attractive, low-growing, aromatic ground cover. Space at 30 to 40 cm apart for ground cover; allow three to four seasons for the colony to develop. Keep consistently moist; water during dry spells in summer to prevent leaf curl and browning. No feeding is needed in moderately fertile soil. Cut back dead and tired growth in early spring (February to March) before new growth begins. Divide congested colonies every four to five years in spring by dividing rhizome sections; replant into prepared, moist soil. Fully hardy throughout the UK. The scent of the dried leaves is most concentrated and pleasant when the cut grass is dried slowly in a warm, airy position; bundles of dried hierochloe can be used as a natural air freshener or aromatic room fragrance.

Q: Is hierochloe odorata native to the UK?

Hierochloe odorata (holy grass) is native to the UK, though it is a rare and scarce native in the UK context; it is found in a small number of sites in Scotland (particularly in Perthshire, Angus, and parts of the Scottish Highlands), northern England, and very rarely in Wales, typically in moist, cool grassland, fen edge, and riverbank habitats. It is much more widespread in Scandinavia, the Baltic states, Russia, and across northern Eurasia, where it grows in large quantities in cool, moist meadow habitats. The UK populations of wild hierochloe odorata are considered scarce and the plant is locally protected in some UK nature conservation contexts. In Poland and other Baltic and Slavic countries, hierochloe odorata (known locally by various names including 'żubrówka' in Polish, from 'żubr', the European bison that is said to favour the grass) is the flavouring used in the traditional Polish herbal vodka Żubrówka, in which a single blade of the grass is placed in each bottle, imparting the characteristic coumarin-vanilla aroma and flavour to the spirit; the coumarin content of the grass means that its use as a food ingredient is restricted in some countries (including the USA) where coumarin levels in food and drink are regulated, though it is permitted in Poland and several EU countries under traditional food use exemptions. In the UK garden context, hierochloe odorata is grown primarily as an aromatic ornamental and ground-cover grass; the scented, dried leaves are a distinctive and pleasantly fragrant garden product.

Hierochloe odorata (holy grass, sweetgrass) is a spreading, cool-season UK native grass notable for the extraordinary sweet, vanilla-like scent of the broad, vivid green leaves when cut or dried. It prefers cool, moist, partially shaded conditions; drought and hot sun cause leaf curl and browning. In the right position it forms a beautiful, low-growing, fragrant ground cover.

Drought and too much sun

The broad, flat, vivid green leaves curl along their length and brown at the margins in full sun in dry soil in a hot UK summer. Hierochloe is a cool, moist grassland plant; drought combined with sun is the worst combination for it. The characteristic sweet, vanilla-like coumarin scent intensifies as the plant concentrates its compounds under drought stress, and the dried stressed leaves emit this fragrance strongly.

What to do

Move to partial shade with consistent moisture; the edge of a light tree canopy, a moist north or east-facing border, or alongside a small pond or stream are ideal. Keep consistently moist in summer; water during dry spells. Mulch with bark or leaf mould to retain soil moisture and keep the root zone cool. In a consistently moist, partially shaded position, hierochloe maintains fresh, vivid green leaves with minimal curl through the UK summer.

Too deep or dry shade

While hierochloe tolerates and prefers partial shade, very deep dry shade causes the leaves to become pale, the vivid glossy green colour to fade to a dull yellow-green, and the plant loses vigour and spreads less actively. In deep dry shade under dense trees, hierochloe struggles just as it does in dry sunny conditions; the key is shade combined with moisture, not shade alone.

What to do

Grow in partial or dappled shade with adequate moisture; dry shade is as damaging as dry sun. If the shaded position is also dry (as under a dense, overhanging tree canopy), mulch heavily with bark or leaf mould to retain moisture and water regularly to supplement what the tree roots take. A position that receives morning or evening sun with shade through the hottest part of the day is ideal.

Too warm and dry in summer

Hierochloe is a cool, northern-climate grass that thrives in the cooler, moister conditions of northern and western UK gardens and in positions that stay cool and moist even in summer. In the warmest, sunniest, driest positions of a UK garden it performs less well than in a cooler spot. It is particularly well suited to Scottish, Welsh, and north of England gardens where conditions most closely match its natural habitat.

What to do

Choose a cool, moist, north or east-facing position in warmer UK gardens; south-facing or west-facing sun-trap positions are not suitable. In a warm UK garden, growing in a container of moisture-retentive compost stood in a cool, shaded position with consistent watering can work well. In a northern UK garden, hierochloe is more at home and requires less special positioning to thrive.

Frequently asked questions

Why are my hierochloe leaves curling?

Hierochloe leaves curl most commonly because of drought and too much sun (cool, moist, partially shaded grassland plant; move to partial shade with consistent moisture; mulch and water in dry spells; warm sun in dry soil causes immediate leaf curl and marginal browning), too deep or dry shade (partial shade with moisture is ideal; dry shade as damaging as dry sun; mulch heavily under trees; water regularly), or too warm and dry a position overall (cool northern-climate grass; thrives in Scotland, Wales, northern England; north or east-facing position in warmer UK gardens). Shade plus moisture is the key combination.

What does hierochloe odorata smell like?

A warm, sweet, vanilla-like aroma with hay-like undertones when the leaves are cut, dried, or crushed; produced by the chemical compound coumarin. Fresh leaves have a mild, sweet, hay-like quality; dried leaves are strongly and distinctively vanilla-sweet. Traditionally strewn on church floors and steps on religious festivals across UK and European history, releasing the scent when walked upon. North American sweetgrass (same species) is a sacred ceremonial plant used by many Indigenous peoples for smudging and basket weaving. Scent is most concentrated and pleasant in slowly dried leaves and bundles.

How do I grow hierochloe odorata in the UK?

Grow in partial to light shade in moderately moist to moist, moderately fertile, neutral to slightly acid soil. Spreads by rhizomes to form a low, aromatic ground cover; space 30 to 40 cm apart and allow three to four seasons to develop. Keep consistently moist; water in dry spells. No feeding needed in moderately fertile soil. Cut back in early spring (February to March) before new growth. Divide every four to five years in spring. Fully hardy throughout the UK. Dried leaves can be used as a fragrant natural air freshener.

Is hierochloe odorata native to the UK?

Yes, but scarce; found in a small number of cool, moist sites in Scotland (Perthshire, Angus, Scottish Highlands), northern England, and rarely Wales. Much more abundant across Scandinavia, the Baltic states, and northern Eurasia. UK populations are considered scarce and locally protected. Famous as the grass flavouring in Polish Żubrówka vodka (a blade placed in each bottle for the coumarin-vanilla flavour). Coumarin content means use as a food ingredient is regulated in some countries. Grown in UK gardens primarily as an aromatic ornamental and fragrant ground cover.