Local plants that are mostly Mediterranean in origin are recommended for Israeli gardens over imported species. This is also true for the Negev region which belongs to the Irano-Turanian climatic zone. There are, however, few introduced plants deemed suitable for gardening. This is mainly because of the fear of these plants establishing themselves within the seed bank and becoming difficult to eradicate in areas that they are not wanted.
I have worked to acclimatise nearly 900 species to Negev gardens of which only 30 are fully
acclimatised historical introductions, i.e. Rosemary, Palm. After 50 years of gardening, of which 29 have been devoted to acclimatisation, only a small number of species have proved themselves to be problematic in terms of their invasive characteristics. These plants include Ruby Saltbush (Enchylaena tomentosa), Blue-leafed Wattle (Acacia saligna), Mediterranean Buckthorn (Rhamnus alaternus) and Ephedra (Ephedra campyopoda). The former two are Australian in origin and the latter two are local Israeli species.
Evidence has recently surfaced that two other species may also be problematic. These are another Australian species - Weeping Acacia (Acacia salicina) and the local Mt Atlas Mastic (Pistacia atlantica).
These have been growing here in Nir Oz since the 1970s but in recent years we have noticed an annual outbreak of seedlings under non-irrigated eucalyptus trees and bougainvillea as well as under irrigated, densely vegetated areas. We have found hundreds of seedlings in these areas as well as in new sites. This is particularly true for P. atlantica. This phenomenon is consistent with the characteristics of highly invasive species.
Over the years we have invested a lot of effort in trying to eradicate R. alaternus and E. campyopoda that have become invasive. With E. campyopoda the process was surprising because of its lag period, for the first 15 years of growth, we didn't find any seedlings. Since then we have seen an enormous annual outbreak of seedlings, similar in habit to the highly invasive Acacia salicina and A. saligna. Pine (Pinus halipensis) also sprouts prolifically but pine seedlings are easy to eradicate and do not constitute a nuisance.
R. alaternus, E. campyopoda and P. atlantica germinate under every bush, even in densely vegetated areas. The problem is that, until one sees the new seedlings, one does not know the scale of the problem and by then the seedlings are well established and hard to eradicate. The solution then requires extensive pruning and application of herbicides (Glyphosate, for example).
I have also observed Eucalyptus gomphocephala, E. gillii and E. spp. spreading along road verges from Sderot to Yad Mordechai on slopes uphill from the main drainage flow. This may well become a problem in future years as they will continue to spread into neighbouring sites.
The problem of P. atlantica is interesting because it is found naturally in northern Israel and is not considered invasive there. However, as it has become introduced to the Irano-Turanic areas of the south it has developed into an invasive species. This problem may be due to environmental tolerance, the removal of competition or a lack of seed predators (birds, mice, gerbils, ants etc). In the gardens that I tend we possibly do not have enough of these seed eaters and the majority of P. atlantica seeds can remain viable in the seed bank. This opens up the opportunity to encourage seed eaters into our gardens as a natural solution to this problem.