Seven years with sorcereled net cages

After the catastrophic winter 2017/2018 When we lost hundreds of trees and bushes to ravenous water sore we have used net cages of galvanized plaster net. The idea is that they the relatively fine-mesh cages should last until the trees have grown out of the most vole-sensitive phase. I described the establishment strategy of which the cages are a part quite detailed 2019 and we still cling to it. In short, it involves the rapid establishment of all trees and shrubs, at the same time as we allow the rest of the land to "regenerate" to arm the soil against water voles, which actually mainly live on grass roots. In the meantime, we are also trying to make the garden more hospitable to the voles' main enemies, such as barn owls and vipers. The fact that we ended up using galvanized plaster mesh was based on one Spanish study according to which the galvanization wears down so quickly that the entire net basket is broken down after 7-10 years.

Mesh baskets fulfill their function

For us, the technology has worked superbly in all three plantations where we have problems with voles and we have not lost a single one of the hundreds of cage-planted trees to water voles since we started using the technology. However, voles have continued to wreak havoc in parts of Puttmyra forest garden, and there have been both sweet cherries, pear, sea ​​buckthorn and apple without protective net basket ironed with. We have also dug up a number of trees to see how the roots manage to get out of the net baskets and to check how fast the decomposition actually goes. No problem there either, the mesh baskets were rusty after two to three seasons in the soil and the roots seemed to have no problem getting out of the baskets.

A few years ago, however, a number of experience reports began to come in from other growers who had completely different experiences with the mesh baskets that I praised. In several of them, the galvanization was completely intact after several years in the soil. Even root spin inside the net baskets described as a problem on social media. However, the vast majority who reported in the various threads on Facebook seem to have had similar experiences to me. How could it be so different?

Corrosion in soil not the same everywhere

One thing I failed to look into when I started delving into the vole protection thing is that corrosion of metal proceeds very differently in different soils. When I started to look at the literature, it turns out that the breakdown of metal in soil is anything but a simple process and that there doesn't even seem to be a working and accepted model for how it happens (Cole and Marnie 2012). Instead, there are a lot of different factors that influence the process, from macro factors such as water table, markfukt, precipitation and so on to various micro-level processes that even I, as an engineer with a whole semester of soil science and a lot of courses in chemistry, do not understand.

Should one try to simplify the whole thing, as far as possible, however, it is the chemical properties of the groundwater, mainly in the form of the pH value, which is most important for the corrosion process (Rossi, Pinamonti m fl 2022). In addition, the degree of compaction of the soil, the temperature as well as the possible presence of chlorides and sulphites are influencing factors. In general, corrosion occurs faster in clay soils and slower in sandy soils.

High and low pH values ​​cause faster corrosion

For the pH value, the lower value applies, the faster the corrosion process. But also the higher the value, the faster the process. Corrosion becomes slower the closer to neutral (7) The pH value is. So if you have good growing soil with a fairly neutral pH, it may be that your net baskets do not break down for many years and that the tree or bush you planted in the basket will eventually have problems with strangled roots. Similarly, in sandy soils with a relatively neutral pH, the decomposition can be far too slow. If you want to find out the pH value of your soil, there are relatively cheap test kits to buy in garden stores. At Puttmyra forest garden where we have soil with a fairly high clay content and a relatively low pH value 5,2-5,4 in any case, we don't have to worry about the decomposition going too slowly. On the contrary, I am beginning to worry that some of our trees and shrubs have not grown enough to withstand future vole attacks.

Root pruning prevents root rot

And the root spin problem then? Here I think it is a matter of poor planting technique rather than a built-in problem with the relatively fine-mesh cages. As I have written elsewhere, among others i Forest Garden and Nut grower's manual, we always do a substantial root pruning before planting, where we cut away the outermost root layer. Then we also wash away part of the nursery soil so that the outer roots have good contact with the soil in the planting pit. After the treatment, all root ends point outwards and the plant can easily find its way out of the cage, provided it is watered properly and regularly and has sufficient nutrients available.

Last but not least, it is important to remember that net baskets should only be used where they are needed! If there are no water voles in your cultivation, you don't need to spend time either, resources and money on net baskets. In our home(forest)garden, for example, we never use net cages. There we have a stony moraine where the voles simply do not thrive.

References

Cole, I. S. and D. Marney (2012). “The science of pipe corrosion: A review of the literature on the corrosion of ferrous metals in soils.” Corrosion science 56: 5-16.

Rossi, S., M. Pinamonti, et al. (2022). “Influence of soil chemical characteristics on corrosion behaviour of galvanized steel.” Case Studies in Construction Materials 17: e01257.