How to Control Winter Grass (Poa annua)
Poa annua annua is commonly know as Winter Grass in South Africa. Although Poa is an extrememly diverse genus, there are two broad classifications : the weakly perennial biotype and the annual biotype. The latter true winter annual is by far the more predominant particularly in the Western Cape and the focus of this article is therefore on this particular biotype.
The Winter Grass Cycle
Winter Grass produces seed in abundance on muliple seed heads and it is this seed which provides the predominant germination in the lead up to Winter i.e. during Autumn when temperatures begin to cool. The Winter Grass then grows healthily in the colder Winter months, particularly in shady areas or where it is damp - amongst lawn grass, in flower beds or wherever it can find a suitable space. As the Winter Grass matures it produces masses of seed, and as temperatures rise in Spring and Summer the Winter Grass goes brown and dies off, leaving behind its seed to continue the annual cycle.
Most experts agree that Winter Grass is best controlled by applying a pre-emergent herbicide known as KERB. Generally KERB should be applied by spraying, starting in early Autumn and during the Winter months when the chances of catching the seed in its pre-emergent germination phase are greatest. For best results it is recommended that application be done when the soil is cool and damp. KERB should only be applied on established lawns. Some Australian experts recommend that lawns be irrigated to the equivalent of 12mm of rainfall soon after spraying and again within 24 hours, presumably to limit or prevent damage to the lawn grass itself. If the infestation of Winter Grass is substantial, then the spraying can be repeated two or three times, at intervals of about two months to the end of Winter. Products for post-emergent spraying might be available but will probably not be necessary.
Since Winter Grass produces so much seed, it is easily spread by wind, water, in compost and lawn dressing, under shoes and on hosepipes. It has a shallow root system and when allowed to grow a bit, can easy be pulled out by hand. This method must in any case be used when Winter Grass is found in not-yet-established lawns, or amongst other plants which might be damaged by KERB residue spray.
Before applying KERB (or any herbicide) CONSULT YOUR LOCAL NURSERYMAN AND/OR THE WRITTEN INSERT INSIDE THE HERBICIDE BOX TO ESTABLISH THE EXACT AMOUNTS AND PROCEDURE TO BE USED.