This invention relates to a polytunnei arrangement, and in particular to a poiytunnel arrangement in which venting of the tunnel can be achieved in a relatively simple and convenient manner.

Poiytunne!s of a range of designs are in widespread usage to provide protection for crops. In some designs the polytunnei comprises a series of rows of upstanding legs, for example screwed into the ground, each leg in one of the rows being interconnected with a corresponding leg of an adjacent row by an arcuate support hoop. A cover is supported by the support hoops to form the tunnel and, typically, is secured in position by ropes which extend over the cover and which are secured either to the associated legs or are otherwise anchored to the ground. Other rigid frameworks may be used to support the cover.

It will be appreciated that the overall shape of the tunnel will depend very much upon the shape of the support structure used to support the cover. Whilst potytunneis provide good protection for crops agatnst wind and heavy rain, and also can result in the crops growing at elevated or optimal temperatures, resulting in enhanced crop growth rates, earlier harvests, better controlled harvesting times, during certain times of the year, and in particular when used in relatively hot countries such as Brazil and Mexico, there is a risk that the temperature within the tunnel will be higher than is desired. In order to reduce the risk of the temperature rising to a level sufficient to cause damage to the growing plants or crops it is desirable to provide the tunnel with vents which can be opened to allow hot air to escape from within the tunnel.

Whilst described hereinbefore in terms of temperature, it may also be important to control humidity within a tunnel, and openable vents can be used to achieve this.

The provision of openable vents on polytunnei type structures has been implemented in the past by incorporating rigid vent frames in the structure which can be moved between a closed configuration and an open configuration in much the same way as a hinged window. Whilst such venting arrangements may operate satisfactorily, implementation thereof is expensive and complex, and so is undesirable. Rather than incorporate a specific vent of this form in a polytunnei structure, it is common for the cover of the polytunnei to be rolled or pushed up, for example using poles or the like to move the cover, to increase airflow into the tunnel. Such a technique may achieve satisfactory airflow, but runs the risk of damaging the cover. It may also require release and subsequent refastening of the anchorage ropes used to secure the cover in position, and so may be a time consuming process.

It is an object of the invention to provide a polytunnei structure incorporating a vent arrangement of simple and convenient form.

According to the invention there is provided a polytunnei structure comprising a support frame, and a cover supported, in use, by the support frame, wherein the cover includes a vent region of net or net-like form, and a cover flap secured to the remainder of the cover adjacent the vent region and movable between an open position in which the vent region is exposed and a closed position in which the cover flap extends over and closes the vent region.

It will be appreciated that simply by moving the cover flap, the vent formed by the vent region can be opened to increase ventilation within the polytunnei, or closed to reduce such ventilation.

The cover is conveniently formed, primarily, of a woven laminated plastics material to which a net material forming the vent region is stitched. The cover flap is conveniently also of the woven laminated plastics material.

Conveniently, the cover flap has ropes secured thereto to, adjustment of the positions of the ropes adjusting the position of the cover flap and so adjusting the ventilation of the polytunnei.

The vent region is conveniently formed at the apex or uppermost part of the polytunnei. However, it will be appreciated that it may be located elsewhere in the cover.

The support frame may take the form of a pair of rows of legs, each leg of one row being interconnected with a corresponding leg of an adjacent row by an arcuate support hoop. It will be appreciated, however, that other forms of support frame may be provided. Depending upon the shape of the support frame, the polytunnel may take a range of shapes including, for example, more traditional greenhouse shapes. The cover conveniently includes a plurality of rope anchorage points whereby anchor ropes can be attached to the cover to secure the cover in position upon the support frame. For example, the cover may have side flaps provided with eyelets whereby the anchor ropes can be secured in position. Such an arrangement is advantageous in that it avoids having the anchor ropes extending completely over the cover and so the anchor ropes do not interfere with the movement of the cover flap.

The invention will further be described, by way of example, with reference to the accompanying drawings, in which: Figures 1a and b are diagrams illustrating polytunnel designs; and

Figure 2 illustrates the cover of an embodiment of the invention which may be used on the polytunnel designs of Figures 1a and 1 b. Referring to Figures 1a and 1b of the accompanying drawings, polytunnels are shown which comprise a support frame 10 supporting a cover 12. The support frame 10, in Figure 1a, is made up of a pair of parallel rows of upstanding support legs 14. Each support leg 14 of one of the rows is connected to a corresponding support leg 14 of the other of the rows by an arcuate support hoop 16. The cover 2 is stretched over the support hoops 16 to form a tunnel of elongate, curved roofed form. The cover 12 is secured in position on the support frame 10 by anchor ropes. As illustrated, a plurality of tunnels may be arranged in a side-by-side configuration, with the legs 14 of some intermediate ones of the rows supporting the support hoops 16 of two adjacent polytunnels. Figure 1 b illustrates a structure in which the legs 14 are omitted and, instead, each support hoop 16 extends to the ground. Such an arrangement will typically be used where a single polytunnel, rather than a plurality of polytunnels arranged side-by-side, is required. The Figure 1a structure could be used either for a single polytunnel or (more usually) where a plurality of tunnels are required. it will be appreciated that the polytunnel structures described hereinbefore are, for the most part, of typical form. Furthermore, whilst the description is of typical structures, a number of other po!ytunnel designs are known, and the invention is equally applicable to those designs. By way of example, arrangements are known in which anchoring techniques other than anchoring ropes, for example so-called zip-lock arrangements in which an edge of the cover is secured within a channel in part of the support frame, are used, and the invention is equally applicable to such arrangements. Furthermore, the polytunnels may be temporary structures, or may be permanent installations having parts thereof mounted permanently in, for example, concrete foundations or anchorages.

In accordance with the invention, the cover 12 is designed in such a manner as to include a vent region 20. The majority of the cover 2 is of a woven laminated plastics material, and the vent region 20 is formed by stitching into the cover 12 a pane! 22 of a net or net-like material. By way of example, it is anticipated that the pane! 22 will be of width of the order of 500 to 1000mm. However, this is merely an example and panels 22 of other dimensions may be used. To some extent the size of the panel 22 will depend upon the environment in which the polytunnel is to be used, and hence the amount of ventilation required, and the requirements of the crop to be grown within the polytunnel.

Adjacent an edge 24 of the vent region 20 is mounted a cover flap 26. The cover flap 26 takes the form of a panel of woven laminated plastics material, for example it may be of the same material as the majority of the cover 12. The dimensions of the cover flap 26 are chosen such that the flap 26 can cover, entirely, the vent region 20. Conveniently, the cover flap 26 is stitched to the remainder of the cover 12. However, depending upon the design of the polytunnel, it may be possible for the cover flap 26 to be formed integrally with the majority of the cover 12. The cover flap 26 is provided with eyelets 28 to which are secured ropes 30. It will be appreciated that by releasing certain of the ropes 30 and tensioning others of the ropes 30, the cover flap 26 can be moved between a closed position (in which it overlies the vent region 20) and an open position (in which the vent region 20 is not obscured). When the cover flap 26 is held by the ropes 30 in its closed position, it will be appreciated that little if any airflow through the panel 22 takes place and so ventilation is low. Adjustment of the positions of the ropes 30 to move the cover flap 26 to its open position permits such airflow to take place and so increases ventilation.

As outlined hereinbefore, controlling ventilation is advantageous in the enhanced control over the temperature and/or humidity within the po!ytunnel can be achieved.

In the arrangement illustrated the vent region 20 is provided at the apex or upper most part of the polytunnel. However, it will be appreciated that in some arrangements it may be desired to locate the vent region 20 elsewhere. Furthermore, additional vent regions could be provided if desired.

Close to the edges of the cover 12 are formed side flaps 32. The side flaps 32 are conveniently of the same material as the majority of the cover 12 and are stitched thereto. Each side flap 32 is formed with a plurality of eyelets 34 forming anchorage points whereby anchorage ropes can be secured to the cover 12 and used to secure the cover 12 in position upon the support frame 10, for example by anchoring the anchorage ropes to the legs 14 or to the ground. It will be appreciated that by using side flaps 32 to which the anchorage ropes are secured instead of having the anchorage ropes passing completely over the cover 12, the anchorage ropes will not impact upon or restrict movement of the cover flap 26.

Where anchoring techniques other than anchoring ropes are used, then there may be no requirement to provide side flaps. Indeed, the side flaps could be omitted in some arrangements where anchoring is achieved using ropes.

It will be appreciated that the invention may be applied both to newly constructed polytunnels and may also be retrofitted to existing polytunnels by replacing and/or modifying the covers thereof. Whilst the arrangement described hereinbefore is of one embodiment of the invention, it will be appreciated that a wide range of modifications and alterations may be made thereto without departing from the scope of the invention.