Annual Poa (Poa annua)
Annual poa, also known as winter grass, annual bluegrass or meadow grass, is one of the most common—and frustrating—grassy weeds found in New Zealand lawns. It’s a fine-textured grass that blends in easily, which is why many people don’t notice it until it starts flowering.
Unlike most desirable lawn species, annual poa is short-lived. It thrives through the cooler months but often dies back in summer once conditions turn hot or dry, leaving bare, patchy areas behind. It also tends to be a lighter green colour, giving lawns an uneven appearance. Because of this, even a small amount of poa can make a well-kept lawn look uneven.
Annual poa germinates whenever moisture and temperature suit—often in autumn, winter and spring—and can produce thousands of seeds per square metre. Those seeds can germinate almost immediately after forming, making it a constant problem once it’s established. Some biotypes behave more like perennials, surviving year-round where lawns are irrigated, so it can be an issue in any season.
Identification
Annual poa is a small, tufted grass with soft, smooth leaves and a distinctive canoe-shaped leaf tip. The colour is a pale, yellowish green compared with the deeper green of ryegrass or fescue.
It often produces tiny white seed heads, even under close mowing, which are one of the first signs of infestation. The seed heads appear throughout the year but are most noticeable in winter and early spring. Leaves may also have faint crinkles partway down the blade, especially on younger growth.
It blends easily into lawns but becomes obvious when the rest of the grass darkens up after fertilising—the lighter poa stands out in patches.
Why It’s a Problem in Lawns
Annual poa grows fast, sets seed quickly and dies back just as fast, leaving open patches that invite more weeds. It thrives in compacted, moist, and shaded areas, and easily takes over thin lawns.
In hot or dry conditions, it browns off and dies, creating uneven colour and bare areas. It’s also more prone to lawn diseases like fusarium and brown patch, especially in humid or poorly drained conditions.
Control in Home Lawns
Poa annua is one of the toughest lawn weeds to control because it’s so similar to desirable grasses. A mix of chemical and cultural control is usually needed. Herbicide options are limited, most broadleaf herbicides won’t touch it.
Best control methods:
Pre-emergent herbicides: The best way to manage poa is to stop it before it germinates. Apply a pre-emergent such as NZLA Blockade in early autumn before the main flush of seed germination. This helps block new seedlings from taking hold.
Selective post-emergent control: For ryegrass and tall fescue lawns, ethofumesate (NZLA Etho) can help suppress existing plants and reduce regrowth, though it’s unsuitable for fine fescue, couch or bentgrass.
Growth regulators: Products containing paclobutrazol (NZLA Paclo) can slow down poa growth and seed production, helping desirable grasses compete more effectively.
Cultural control: Reduce watering through summer, avoid excessive phosphate fertiliser, and keep soil aerated. A dense, healthy lawn will shade out poa seedlings and make it harder for them to establish.
In garden beds or paths, non-selective herbicides such as glyphosate are effective.
In Fine Fescue lawns haloxyfop (NZLA Halo) gives the best results. Avoid using the same active repeatedly to prevent resistance, some poa populations have developed tolerance to haloxyfop. Do not use NZLA Halo on tall fescue or ryegrass.
Prevention Tips
- Apply a pre-emergent herbicide each autumn to break the germination cycle.
- Improve soil drainage and reduce compaction through annual aeration.
- Maintain even fertiliser coverage to prevent thin, stressed patches.
- Avoid overwatering—poa thrives in constantly damp soil.
- Mow regularly and at the right height for your grass type to encourage dense growth that shades out poa seedlings.