Not only on or along the bridges is the supply of the plants ensured, but also more distant parks and green strips in the city center are co-vitalized by the bridge. The bridges bring the water to the city districts for this purpose, and from there the fine distribution takes place partly via tankers, partly with the help of further pipelines in the ground, or also in parks via watering water geysers connected to the bridge system.
For inner-city trees, there is surge irrigation with a tanker or connection to rain gutters of neighboring buildings.
Beds are also connected to rain gutters and also equipped with a clay pipe system that can also be filled with the tanker truck like a reservoir.
In Frankfurt's city parks, on the other hand, water tower-like columns are installed to distribute water like geysers at night during dry periods.
Irrigation of plants is part of a concept that aims to turn Frankfurt into a "sponge city". Thus, in the city of the future, rainwater should not be discharged into the sewage system but, wherever possible, collected and deliberately kept in the city by irrigating urban greenery with it.
Since trees get most of their water needs from the soil and groundwater, watering them in March and July with 1000 liters of water in one burst is enough to get them through the summer. Most mportant: The trees need a kind of catch basin that can be filled quickly and then let the water slowly seep into the ground.
The catch basin should be 25cm high and only 10cm deep in the ground: This is because no concrete work should be done on existing trees. The border should be made of natural materials from the region - in Frankfurt, for example, Taunus quartzite would be suitable.
If it becomes apparent at the end of August that the fall will also be rather dry, the tree can be shock watered a third time at the end of September. Tankers can always fetch the water at the nearest tap of the bridge ring main. This eliminates the need for lengthy water transport trips.
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Every tree in the city should have at least one to two square meters of planting around it: That adds up to another 10,000 m2 of planted area in the city.
Underplanting protects against dirt and damage, retains moisture and promotes natural decomposition processes in the soil.
However, it should only be provided for trees that are already established on the site to avoid competition for water and nutrients.
While trees can be shock irrigated, green areas without trees but smaller plants, need water more regularly.
Earlier generations already knew a sustainable solution for this: Clay cones in the ground are filled with water and, as "mini reservoirs," continuously release this water to the plants.
To fill these reservoirs more efficiently than is done in private gardens with clay cones, a clay pipe system can be laid in the ground instead of individual cones. This system can either be connected to rain gutters or filled by tankers during extreme dry periods.
The water slowly seeps through the clay into the bed. Because the last filling of the pipes takes place in September, they can also be laid near the surface of the earth, because they lose enough water until the frost period, so they will not be damaged during the winter period.
In the course of pipe laying, compacted and contaminated soil can be removed and replaced: This also contributes to the vitalization of the plants there and also offers - at least in "clean" streets - the possibility to favor the infiltration of street water into the beds by light trough shape.
The "night geysers" were developed for this purpose: They are built similarly to small water towers, about 2.20 m high and equipped with water nozzles. In dry phases they (controllably) eject water in high arcs in different directions, and for the park this is like a heavy rain event after midnight, only without thunderstorms.
Inner-city parks serve as a space for recreation and are invaluable for the urban climate. In dry years, however, the plants there suffer because irrigation is historically not provided for such large green spaces.
The columns are designed like small water towers, so that they visually enhance the park. These "water tower columns" must be placed in parks so that they reach as many trees as possible and at the same time do not get in the way of recreational activities such as soccer, etc.
The irrigation intentionally imitates night rain: although it is strictly speaking a "surge irrigation", it is de facto carried out over several days in smaller quantities. The soil can thus absorb the water better, no waterlogging collects under the trees, the surrounding smaller plants also get some, and even the animals in the park can quench their thirst in the morning after the "rainy nights".
The water geysers have many nozzles, through which with high pressure large amounts of water can be applie, preferably at night. The range as well as the amount of water is variably adjustable. The duration of the rain should be as long as possible, so that the soil can soften and the water can reach the roots successively. With moderate amounts of water being distributed over longer time periods, animals are also protected from drowning. The aim is to wet the soil over a large area so that trees can also develop deep root growth.
The vitalization of the city's greenery works similar to the irrigation of the newly unsealed areas in Frankfurt: Through direct connection to the irrigation system of the Frankfurt‘s Bridges or through their numerous water taps for tankers, large areas in Frankfurt can be irrigated with collected rainwater.
In addition, there are special concepts such as shock irrigation of street trees at certain times of the year or night rain showers in parks.