Digitaria

Digitaria

Crabgrass
Binomial:	Digitaria spp.
Seed Dispersal:	Dropped, or on tools and clothing

Digitaria is a genus of about 300 species of grass (family Poaceae) native to tropical and warm temperate regions. Common names include crabgrass, finger-grass, and fonio. They are slender monocotyledonous annual and perennial lawn, pasture, and forage plants; some are often considered lawn pests. Digitus is the Latin word for "finger", and they are distinguished by the long, finger-like inflorescences they produce.

Description

All crabgrasses have similar growth habits and flowering structures, but species are separated by minor differences in the flower structures and leaf pubescence. They typically have spreading stems with wide flat leaf blades that lay on the ground with the tips ascending. The inflorescence is a panicle in which the spike-like branches are arranged in digitate fashion. The spikelets are arranged in two rows on an angled or winged rachis. Each spikelet has two florets, only one of which is fertile. The first bracts at the base of the spikelets are either very minute or absent.

Ecology

Crabgrass seed has a long germination period; if conditions are right, it can germinate throughout the growing season. Crabgrasses occur in tropical, subtropical, and temperate regions of both hemispheres.

The most prevalent species of Digitaria in North America are Large Crabgrass (D. sanguinalis) and Smooth Crabgrass (D. ischaemum), which often become problem weeds in lawn and gardens. They are annual plants, and one plant is capable of producing 150,000 seeds per season. The seeds germinate in the late spring and early summer and outcompete the domesticated lawn grasses and expand outward in a circle up to 12 inches in diameter. In the fall when the plants die they leave large voids in the lawn. The voids then become prime areas for the crabgrass seeds to germinate the following season. Crabgrasses also have a different texture and color that often interrupts the uniformity of a lawn. In vegetable gardens, crabgrass can quickly out-compete desirable plants, causing yield reductions.

Uses

Despite their weediness, crabgrasses do have a few redeeming qualities. The seeds, most notably those of fonio, can be toasted and ground into a flour, which can be used to make porridge or fermented to make beer. Fonio has been widely used as a staple crop in parts of Africa. It also has decent nutrient qualities as a forage for cattle.

Control

• Pre-emergents (synthetic): Crabgrasses as controlled with pre-emergent herbicides that interfere with a key enzyme, that is only active when a seed germinates. These herbicides must be applied at a critical time. If they are applied to the soil too early, they get washed too deep into the soil by rainwater. If they are applied too late the key enzyme inhibited is no longer active. The rule of thumb is to apply when the local forsythia blooms are wilting.

References

• Andrea S. Vega and Zulma Rúgolo de Agrasar, Morphological interpretation of the spikelet in Digitaria atra (Poaceae: Panicoideae: Paniceae) and emended generic description American Journal of Botany (2001)