Italian ryegrass (Lolium multiflorum) Identification – Leaves hairless with glossy underside, and tend to be wider than perennial ryegrass. When the stem is cut horizontally the leaves can be seen to be rolled in the sheath. The stems are round (perennial ryegrass is flattened) and auricles are present and without hairs (wheat auricles tend to have hairs). The seedhead is a flattened spike with spikelets arranged alternately on opposite sides of the stem. The spikelets are awned and stalkless and have their rounded face next to the stem. (The spikes of couch are similar but have the flattened face of the spikelets next to the stem.)
Perennial Ryegrass (Lolium perenne) Identification – Leaves hairless with glossy underside, and tend to be narrower than Italian ryegrass. When the stem is cut horizontally the leaves can be seen to be folded in the sheath. The stems are flattened (Italian ryegrass rounded). Auricles are present and hairless. The seedhead looks like Italian ryegrass except that there are no awns on the spikelets.
Biology – Ryegrasses have a competitive index of 0.85, compared with 1.0 for wild oats and 0.4 for blackgrass, so even moderate populations can have severe effects on crop yield. Flowering time is much more variable and later types may not have shed before harvest. Seed can become dormant in the soil and persist for two years and exceptionally for four years.
Italian ryegrass can be annual or biennial, 30-90 cm high. It can be very well tillered or not depending on crop competition, but is very aggressive in its spring growth pattern. Flowering occurs in May-August and seed is shed before harvest and can persist in the soil for up to seven years.
Perennial ryegrass is a competitive weed, but less so than Italian ryegrass. As the name indicates it is a perennial plant 30-60 cm tall and will tiller as much as conditions allow.