Plant problems

Why Are My Digitaria Leaves Curling?

Q: Is digitaria a weed in the UK?

Digitaria sanguinalis (hairy crab grass) is regarded as a widespread annual weed grass in UK gardens and cultivated ground, present across much of England and Wales in gardens, vegetable plots, allotments, borders, paths, and disturbed ground in warm summers. It is classified as an archaeophyte (a plant introduced to the UK before 1500 AD, likely with early agricultural activity) rather than a native species. Digitaria ischaemum (smooth crab grass) is also present but less common. Both species are warm-season annual weeds that germinate from late spring, grow vigorously in summer heat, and are killed by the first autumn frosts; the key ecological feature of both species in the UK context is that they are warm-season (C4) grasses that thrive in conditions where the cool-season grasses of the UK lawn (perennial ryegrass, fescue, meadow grass) are under heat stress in a warm summer — this gives digitaria a competitive advantage over lawn grass species during the hottest weeks of a UK summer in areas where the lawn is thinning, bare patches are present, or the soil is disturbed. Digitaria is not a significant agricultural weed in UK arable farming in the way it is in parts of continental Europe, North America, or tropical and subtropical agriculture, because UK summer temperatures are generally too cool and variable for it to establish and compete effectively in a cereal crop; it is primarily a garden and horticultural weed rather than an arable field weed in the UK context. On warm, sandy, disturbed, or freely draining soils in southern England in warm summers, it can establish in lawns, paths, and bare garden soil and compete strongly with desired garden plants in that specific microhabitat.

Q: How do I control digitaria in a UK garden?

Controlling Digitaria sanguinalis (hairy crab grass) and Digitaria ischaemum (smooth crab grass) in a UK garden relies primarily on cultural practices that prevent germination and establishment, with physical removal of established plants before seed set. Preventing germination: digitaria seeds germinate when soil temperatures warm above 15 to 18°C, typically from May or June in the UK; maintaining a dense, thick mulch layer (5 to 7 cm of bark chip, wood chip, or green waste mulch) in borders and beds prevents light reaching the soil surface and greatly reduces digitaria germination; maintaining a dense, tall lawn sward (mown at 40 to 50 mm) similarly suppresses digitaria germination in the lawn by outcompeting seedlings for light before they can establish; reseeding or turfing any bare areas in spring before the soil warms prevents digitaria from colonising bare ground in summer. Physical removal: young digitaria seedlings can be hoed off easily in cool, dry weather before they have rooted extensively; once established, the spreading root system and multiple spreading stems make the plant more difficult to remove whole without breaking; pull or hoe established plants before the seed heads (the distinctive radiating finger-like branches of the digitaria seed head, which gives the genus its name and its common names crab grass and finger grass) have matured and shed; a single large digitaria plant can produce thousands of seeds in a season. Herbicides: in a lawn, selective pre-emergent herbicides are the most effective approach in countries where they are available; in the UK domestic garden and lawn context, selective herbicides for digitaria control in grass are of limited availability and are not commonly used; post-emergent control of established digitaria in a lawn without killing the lawn grass is difficult with the herbicides available to UK amateur gardeners.

Q: What does digitaria sanguinalis look like?

Digitaria sanguinalis (hairy crab grass) can be identified in a UK garden by the following features: the plant is a warm-season annual, germinating from late May or June and forming a spreading, mat-like plant with branching, prostrate to ascending stems (culms) that root at the nodes where they touch the ground, gradually forming a broad mat that can reach 60 to 90 cm across in a good UK summer; the leaves are flat, broad (5 to 10 mm wide), softly hairy on both surfaces and on the leaf sheaths, giving the entire plant a distinctly soft, velvety or hairy texture that is quite different from the smooth, glossy leaves of many UK lawn grass species; the leaf blades are medium green, broadly lance-shaped, and clearly different in texture and colour from the fine, smooth leaves of the lawn fescues, ryegrasses, and meadow grasses around them; the seed head is highly distinctive and unmistakable: it consists of two to ten finger-like, slender, spreading branches (racemes) arranged in a whorled or digitate (finger-like) pattern at the top of the stem, giving the plant its common name of 'crab grass' or 'finger grass'; these racemes are typically purplish-tinged when young, ageing to reddish-brown, and carry small, oval spikelets in pairs along their length; the seed head is produced from July onwards and is the main identification feature in late summer and autumn; a lawn or border in late summer with digitaria present has the characteristic appearance of the low, spreading mats with the distinctive radiating finger-like seed heads projecting above the surrounding surface at intervals across the area. Digitaria ischaemum (smooth crab grass) is very similar but the leaf sheaths and leaves are smooth or only sparsely hairy rather than distinctly velvety; it is less common in the UK than D. sanguinalis.

Digitaria sanguinalis (hairy crab grass) is a common warm-season annual weed in UK gardens; identifiable by its spreading mat of soft, velvety, hairy leaves and the distinctive radiating finger-like seed heads produced from July. Leaves curl and yellow in cold temperatures, drought, or at the end of the growing season when the plant dies with the first frosts.

Cold and autumn senescence

Digitaria is a warm-season C4 annual killed by the first autumn frosts; as temperatures drop in September and October, the leaves curl, yellow, and the plant collapses and dies rapidly. This sudden yellowing and leaf curl in early autumn is the normal end-of-life of the annual and is not a treatable cultural problem. In a UK garden, the plants have usually set seed well before they die, so removing them before frost does not reduce the seed burden for next year.

What to do

If managing digitaria as a weed, remove plants before the finger-like seed heads mature (before late August to September); a single plant can produce thousands of seeds in a season and the seeds persist in soil for several years. Pull or hoe in dry weather; the spreading mats root at the nodes and are easier to remove whole when the soil is dry. The annual die-back in autumn is inevitable; focus management effort on preventing seed set rather than on treating the leaf curl that signals the plant's end-of-life.

Drought stress

Although adapted to warm, often dry and disturbed conditions, digitaria leaves still curl and wilt in extended hot, dry spells; the soft, hairy leaves are particularly prone to afternoon wilting in hot sun on a dry soil. The plant typically recovers overnight as temperatures drop, but prolonged drought weakens it. This drought curl is more diagnostic of a cultural problem when it occurs in a plant being grown ornamentally rather than as a weed.

What to do

For ornamental cultivation (digitaria is occasionally grown in cutting gardens for its elegant finger-like seed heads), water regularly in dry spells and grow in a freely draining but reasonably moisture-retentive soil. In a UK cutting garden or ornamental border, the afternoon wilting is primarily cosmetic and the plant recovers overnight; the seed heads are the main ornamental feature and are not significantly affected by mild drought stress in most UK conditions.

Establishment failure in cool conditions

Digitaria requires warm soil temperatures (at least 15 to 18°C) to germinate reliably; in a cool, wet UK spring or early summer, seeds may lie dormant for weeks longer than expected, and seedlings that emerge in cool conditions are often weak, slow to establish, and susceptible to damping off or slow-growth curl. As a weed, this means digitaria is less problematic in cool, wet UK summers; as an ornamental, it means reliable establishment requires patient sowing timing.

What to do

For ornamental use, do not sow outside before late May; wait until the soil temperature has reached 15 to 18°C. Start under glass in April if a longer season is desired. For weed management, the warm-soil germination requirement means that maintaining a thick mulch layer (5 to 7 cm) or a dense lawn sward prevents most digitaria from establishing; seeds cannot germinate in cool, shaded soil under mulch or dense turf.

Frequently asked questions

Why are my digitaria leaves curling?

Digitaria leaves curl most commonly because of cold temperatures and autumn senescence (warm-season C4 annual killed by first autumn frosts; sudden yellowing and collapse in September to October is normal end-of-life; manage as a weed by removing plants before seed heads mature in August to September), drought stress (soft, hairy leaves wilt in hot afternoon sun; plant recovers overnight; water regularly in dry spells for ornamental use), or establishment failure in cool conditions (requires soil temperature of 15 to 18°C; do not sow outside before late May; cool-soil seedlings are weak and susceptible to damping off; thick mulch prevents weed germination). Prevent seed set to reduce the soil seed bank.

Is digitaria a weed in the UK?

Yes; Digitaria sanguinalis (hairy crab grass) is a widespread annual weed in UK gardens, vegetable plots, allotments, borders, and disturbed ground, particularly in warm summers on sandy or freely draining soils in southern England. An archaeophyte (introduced before 1500 AD); warm-season C4 grass that thrives in summer heat when cool-season lawn grasses are under stress. Not a significant arable field weed in the UK (too cool for reliable crop-field establishment), but a vigorous garden and horticultural weed in warm summers.

How do I control digitaria in a UK garden?

Prevention: maintain a thick mulch layer (5 to 7 cm) in beds to block germination; maintain a dense, tall lawn sward (40 to 50 mm) to suppress seedlings; reseed bare ground in spring before soil warms. Physical removal: hoe young seedlings before root system establishes; pull or hoe established plants before seed heads mature (before late August to September); a single plant produces thousands of seeds. Herbicides: UK availability of selective pre-emergent and post-emergent digitaria herbicides for domestic use is limited; cultural control is the practical approach for most UK gardeners.

What does digitaria sanguinalis look like?

Spreading, mat-forming annual with branching, prostrate to ascending stems that root at nodes; mats to 60 to 90 cm across in a good UK summer. Leaves flat, broad (5 to 10 mm), softly hairy (velvety) on both surfaces and on the leaf sheaths; distinctly different from smooth lawn grass leaves. Medium green, broadly lance-shaped. Seed head highly distinctive: two to ten slender, spreading, finger-like branches (racemes) in a whorled or digitate pattern at stem top (hence crab grass and finger grass); purplish-tinged when young, ageing reddish-brown; produced from July. Digitaria ischaemum (smooth crab grass) is similar but with smooth or sparsely hairy leaves; less common in the UK.