The Hague in the Netherlands

The City and its elm population

The Hague, located in the province of ‘South Holland’ along the North See coast, is the city where the Dutch Government has been seated since the Middle Ages. Out of a total population of approximately 100.000 trees in the city district, 20.000 alms are located along streets and canals and another 10.000 prevail in parks and semi-forest stands, most of them under care of the City.

The elm represents 30% of all street trees in the city. Close to the coast, the elm is one of the few trees, capable of withstanding the salty see winds and still grow well in the harsh conditions for tree growth in city circumstances. On top of that, this tree species has been a part of Dutch history in the city for centuries, being a very characteristic tree in the city panorama.

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Dutch elm disease in The Hague

The city started searching for a solution for the Dutch elm disease disaster in the 90's. At that time the city was losing thousands of elms in their streets yearly to Dutch elm disease, approximately 7% out of the total every year (see graph 1). This plague was not limited to losses of old and valuable city elms only, but also involved high costs for removal and replacement of the diseased street trees every year.

The Dutch Trig® injection and sanitation program

In 1995, 1996 and 1997 the preventative Dutch Trig® treatment was introduced and assessed to determine the usefulness and effectiveness of this biological treatment this area. The results of the treatments were very satisfactory, showing a big decrease in Dutch elm disease prevalence in the treated street and park trees. These positive results have led to the set up of a large scale injection scheme in 1998 by the City's Park Service, selecting about 8.500 out of 30.000 trees to be injected from that year onwards.

Because it was not possible to inject all elms due to the limited budget, the selection focused on trees closest to the coastal line, along main roads, canals, market places and (old) trees with an historic value (monumental trees). This injection scheme was extended with another five hundred trees in 1999 and comprised over 10.000 elms in 2000.

Together with the injection scheme a sanitation program was reinforced. This meant that elms in the city would be checked for Dutch elm disease several times during the growing season, diseased elms and all elm-fire-wood removed and disposed of rapidly and lost elms prudently being replaced, only with more resistant elm clones.

Treatment results

The results of these actions as presented in graph 1 show that the disease incidence in The Hague started to decline considerably, from the moment the large-scale Dutch Trig® injection scheme was implemented.

The fact that a Dutch Trig® injection does successfully prevent Dutch elm disease, is illustrated by the fact that disease incidence shows an immediate drop of disease incidence from 7 % in the overall- group to 0.7 % in 1998, 0.3 % in 1999 and only 0,16% in 2000 in the group Dutch Trig® treated group.

Even though only 10.500 elms out of 30.000 city elms in the city are treated with Dutch Trig®, a drop in the overall disease incidence is visible. This drop is most probably caused by the low disease incidence in the Dutch Trig® treated group (of which results are also incorporated in the overall disease incidence).

Successful combination of Sanitation and Preventative Treatment

Sanitation alone can not prevent an elm from dying of Dutch elm disease. The above illustrated successful reduction in disease incidence is caused by the combination of the sanitation efforts and the injection program in the Dutch Trig® treated group of elms. If only Dutch Trig® injections would be administered, without proper sanitation practices, the disease incidence would be higher, because root graft transmitted infections would rapidly kill large numbers of trees surrounding the diseased tree, even if they have been treated with Dutch Trig®.

Trends in disease incidence

The trees that were removed due to Dutch elm disease in the Dutch Trig® treated group (58 out of 8539) the first year of the City's injection scheme, were almost all lost due to old Dutch elm disease infections in previous years growth rings (old infections) and root graft transmitted infections. Research into the tree's history, using microscopic assessments of the tree's growth rings, show vascular discolorations caused by Dutch elm disease infections in previous year's growth rings.

In City's where the Dutch Trig® treatment is administered for the first time, disease incidence is always higher when compared to the disease incidence in a city which has the injection program running for several years. This difference is caused by the fact that in the first year, already infected trees, showing no symptoms yet, will be injected and will still be lost to Dutch elm disease. If the injection program is continued, the already diseased trees will be 'filtered out', resulting in the steady drop of disease incidence in the injected group, because no new diseased trees arise in the Dutch Trig® treated group.

In the case of The Hague, with a large number of the already infected trees being filtered out in 1998, the number of trees lost in 1999 was reduced to only 27 trees out of almost 9000 and even further down to only 17 trees (of which 15 were root graft transmitted infections) out of 10.665 treated trees in 2000.

Public interest and involvement

Besides the injection and sanitation program, the public action "Heart for a Tree" was set up in 1997 by the City's Park Service to stimulate inhabitant's interest in and awareness of city trees and their value. This initiative arose a big response, with a large number of inhabitants volunteering to assist in monitoring the city trees and reporting damages, pests and diseases.

The decision of the city to protect such a large number of elms from 1998 onwards was received with great joy by many inhabitants. Street-Tree-Action-Committees arose in several neighborhoods as a result of the "Heart for a Tree" action. These committees even managed to gather the money privately, needed to extend the injection program in, or into, their streets with the goal to protect 'their' elms from Dutch elm disease.

Conclusion: a green and characteristic The Hague

The Hague aimed at reducing the loss of valuable city elms, by setting up the sanitation- and injection program and generating public interest for city trees. Despite all the ongoing discussions on the usefulness of elm protection and injection practices, the results show a significant decline in the number of trees lost to Dutch elm disease. The city therefore continues and elaborates its effort to preserve elms with all available tools, supported by arborists, the public and politicians, all enjoying a 'green' and characteristic The Hague.