POLES FOR VINEYARDS: WHERE METAL IS UNBEATABLE

The trellising systems now being used in state-of-the-art winemaking are at last being deployed with confidence, after many troubled years of uncertainty. The process of updating the vine-training systems utilised by winemakers, ongoing since the late 1980s, has taken more than 20 years to arrive at what is the ideal material from which to make posts for a modern vineyard. A wide array of different experiments conducted over the years – encompassing pre-stressed concrete, treated wood and various types of plastics ­– have highlighted the pros and cons of the materials in question.

Today, in an awareness that the ideal stake must be elastic, resistant and durable, the choice faced by winemakers is a proverbial “no brainer”, since these are the properties of metal poles – steel poles, to be precise. The consolidation of the latest solutions used for vineyard-post systems has, in turn, conditioned the selection of the type of wire, with a view to meeting, first and foremost, the requirement for practicality of installation, but also to achieving the correct tension.

METAL POLES: WHAT AND HOW TO CHOOSE?

For many winemakers, the most popular vineyard trellising systems – which have been in use for more than 15 years and, as such, have been widely tested – are those based on metal poles. These vineyard trellis posts may be of various shapes, sizes and thicknesses, using different types of steel and featuring different cut-outs. The popularity of metal vineposts due to their specific features (see here) has simplified to a great extent the choice faced by winemakers – indeed, today, metal is practically taken for granted as the only viable option. Nevertheless, appropriate evaluations should still be made in relation to the type of steel (whether certified or uncertified), as well as to the shape of the stake and its thickness.

TYPE OF STEEL

Sika metal trellising is rustproof and, therefore, able to resist both the humidity of the soil and contact with fertilisers and pesticides. Resistance to corrosion is essential in order to give the trellis post a useful lifespan that is at least as long as that of the vineyard itself. The materials that have been thoroughly tried and tested over the years are hot-dip galvanised steel (i.e. with galvanisation carried out post-forming), pre-galvanised steel (where the stake is obtained from galvanised sheet steel), and Cor-ten steel.

Galvanized steel

Example of the oxidation of Cor-ten over time.

The real turning point for vineyard trellising systems came with the introduction of Cor-ten steel. Sika was one of the first manufacturers in Italy to utilise this material, which is now by far the most popular choice amongst winemakers. The reasons for this success are as follows:

  • Lower cost: for the same profile and thickness, Cor-ten steel vineposts cost on average 20% less, although recently Cor-ten has been subject to price increases as a result of issues relating to supply and demand.
  • Resistance to corrosion: Cor-ten steel – the patented name is actually a conjunction of corrosion resistance (COR), and tensile strength (TEN) – undergoes natural passivation by atmospheric agents. This makes the metal turn rust-coloured and allows the stake to protect itself without compromising its properties of resistance.
  • Environmental mitigation: the “rust” colour, which is in fact closer to wood than to rust, enables Cor-ten trellising to blend in with the environment to a far greater extent, and as such this material is also much appreciated for its aesthetic qualities.

Sika cold-roll formed poles are without doubt the most popular grape post, thanks to the ease with which wires can be fed through the slots and hooks (from 12 to 14). There are a large number of different types of profiles available on the market, due to the design choices made by the various manufacturers. However, in contrast to what we have been led to believe, the characteristics of the ribbing play only a secondary role in terms of resistance – the thickness and width of the section being of greater importance. The strips of sheet steel are cold-rolled to produce the stakes, and then slots of various shapes and sizes are cut out; the slots are generally positioned at regular distances along the upper part of the profile of the pole. The most widely used type of slot is that which is cut out from within the profile – i.e. it does not protrude from the section of the stake itself and thus does not put the wire at risk of being crushed during mechanical harvesting. Most of the slots are cut out in such a way as to allow the wire to be fed through prior to its tensioning, which in turn is the operation that prevents it from becoming unhooked. The shape of the slot must ensure that the wire is not then subject to crushing or scraping. Normally, the various manufacturers of trellising make their trellis posts with slots of a single type (N-H-U); only SIKA stakes for vineyards feature H + N + H combined cut-outs.

THICKNESS

In terms of the level of resistance it offers, the thickness of the grape stake remains crucial, together with the width – even more so than the section. With Cor-ten steel vineyard trellising, the weight of the material does not affect the galvanising process; indeed, increased levels of thickness have been successfully utilised, even up to 1.9/2.0mm, thus enhancing the resistance of the profiles to a considerable extent.

THE RESISTANCE OF THE VINEPOSTS

Normally, a steel trellis post has a resistance of just 1/3 with respect to that of wood, and 1/2 that of concrete. For this reason, over and above the fact that the structures used in the past were probably oversized from this perspective, with the introduction of metal stakes the distance between the poles along the row has decreased, with around 25% more vineposts being installed, improving the linearity and tensioning of the main wire in the process. The trellis is further strengthened by placing the stakes closer together on the outer rows, which are most exposed to the wind. In practice, this involves substituting, on the external rows, a brace with an additional stake that is identical to the others, thus not affecting mechanical harvesting in any way (the same cannot be said for concrete). If the rows are very long, one effective strengthening solution can be to insert additional stakes at regular intervals, transversally aligned vis-à-vis the vineyard (i.e. perpendicular to the direction of the row), substituting as before a brace with a vinepost. It should be stressed, though, that over the course of recent years we have also witnessed a notable increase in the sections and thicknesses of sika profiled vineposts, both Cor-ten and galvanised, rising from thicknesses of 1.5mm to a minimum of 1.8mm.