Poa Aquatica: Its Uses And Its Dangers

New Zealand Journal of Agriculture, Volume 72, Issue 4, 15 April 1946, Page 339

Poa Aquatica: Its Uses And Its Dangers

By

C. WALKER,

Instructor in Agriculture, Thames.

TN certain circumstances some plants are valuable; *in others they may be harmful. This is well illustrated by a plant, the botanical name of which is Glyceria aquatica. This plant until a few years ago was called Poa aquatica, and it is by the latter name that it is probably better known. Both the value of Poa aquatica as a fodder plant and its danger as a plant that will block streams relation to its present distribution in the Thames Valley basin— are discussed in this article.

AT present there is quite a sharp divergence of opinion in the Thames Valley basin on the question of whether . Poa aquatica. has a place in New Zealand’s farming economy or whether it is a distinct menace to farm lands.

This controversy has been provoked l by the fact that the grass, since .- its establishment in the district, has spread at an alarming rate into waterways, and has seriously invested drains, canals, and even rivers. The effect upon the waterways is marked, particularly in checking ■ the flow .of water, in causing much silting up, and in completely eliminating waterways in some places by growing right across them. County, councils’ officials, drainage boards’ authorities. and farmers in the vicinity have been viewing the position with considerable alarm; they have been agitating for drastic action to be taken in an effort to combat the grass. Viewed from this angle, the spread of the grass is serious; some action is necessary to ensure that waterways are kept open. The grass is so vigorous in its rate of growth that in a few years a small colony will develop into an area of several acres. Moreover, with its ability to reestablish from sections that break off and float about, the grass can spread very rapidly along water- ' ways. In tidally-affected canals long lines of this material are commonly seen moving up and down the streams according to the flow of the water. In due course these pieces become ' anchored to form new colonies, or they may be carried by floods across country to establish on wet peat land.

Value for Grazing Despite adverse criticism of the grass because of the trouble caused in waterways Poa aquatica is generally conceded to be of value for stock grazing. This is borne out by the fact that one farmer devotes nearly 100 acres of his 320-acre farm to this grass. The land in question is partlydrained shallow peat which was' first sown in ..the grass about 15 years ago. The area then was poorly drained and

largely covered by rushes; there were large clumps of blackberry on the better-drained parts. In the intervening period Poa aquatica spread over the whole 100 acres, developed a very dense sward, eliminated blackberry and rushes almost entirely, and today is fattening bullocks. The farmer who is handling this area

claims that he can carry an average of a grown beast per acre throughout the year, and . for much of this time he is compelled to double this rate of stocking in order to hold the growth. His system of drainage is to provide shallow depression channels rather than conventional drains. .. These simply provide for collection of storm water. which they convey to main waterways. ■. At the same time ' they do not lower the water-table, sufficiently to enable English grasses and clovers to flourish, although in parts he had an appreciable amount of Lotus major showing.., An examination of this farmer’s stock "supported his claim that they do well on the grass; and, in these circumstances, it must be concluded that the grass here is serving a useful purpose.

Spread of the Grass The problem, however, is not the establishment of Poa aquatica on areas where it is likely to be suitable. Rather, it is a matter of controlling it and of preventing it from spreading to waterways that serve other highlyproductive lands.

The grass is a menace only because it tends to block streams. It is not regarded as likely to harm other pasture lands by actual invasion, as it will not tolerate good drainage conditions for long. :

The spread of Poa aquatica occurs in one or more of the following ways: 1. By stock grazing the edges of drains and streams, and breaking ' off pieces which float away to establish elsewhere; 2. By drain cleaners failing to remove all pieces of Poa aquatica from the streams while they are at work;

3. By large rafts of material which break off from parent colonies floating away and anchoring elsewhere; j- . • 4. By floods carrying pieces across country; and 5. By wild fowl (probably duck) carrying seed from place to place. Where possible, ’control should be exercised, as, for instance, by keeping stock from affected drains, or by advising drain cleaners to put all material on banks. . Apart from this, however, the major problem of keeping large waterways open and fairly free from the grass requires the attention of drainage engineers .working through some competent, authority. Dredges would be necessary, and, in such work, care should be exercised to ensure that no grass escapes to. establish other colonies. The bulk of the grass should be placed on high land where it will die. Fortunately most of the rivers and canals are provided with stop banks upon which the Poa aquatica could be placed as it is dredged from the streams.

Other Means of/Control

z Apart from the controls already advocated, the planting of drains and canal banks with trees so that the banks are well shaded has the effect of greatly , reducing grass establishment. Farmers and drainage boards, therefore, could assist greatly in control of Poa aquatica by planting their waterways with trees to reduce the growth, of the grass. ’ ■

Owing to the huge bulk of water in streams recourse to poison sprays would not be very effective. Sprays would be quickly diluted to a strength which would be quite innocuous. However, in tidally-affected streams Poa aquatica fails to establish or thrive where the water is saline. Consequently no acute problem exists in the lower reaches of drains, streams, and rivers where salt water is encountered, although the grass menaces much of the drainage system of these areas, and , fairly strong control measures are required. - It is not anticipated that any r measures will be completely successful for eradicating the grass, but maintenance of waterways by dredging would minimise its present adverse effect upon land drainage. While the grass has certain virtues on poorly-drained land where it will establish freely and carry a quantity of stock equalled only by first-class pastures on more favoured country, the main consideration should be that of the greatest good for the largest number; and, if this point is conceded, then Poa aquatica will require rigid control to prevent it from becoming a menace to other farm lands.'

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