Bonn Falling leaves, split bark and yellow lawns – the heat and drought have left their mark. Cornelia Löhne, scientific director of the Botanical Gardens, explains how gardens can recover from the extreme weather.
The effects of the heat wave over recent weeks in Bonn are visible in every green space. The civic gardeners have watered the beds and trees in the city area with the support of the voluntary fire brigade and the THW. Employees in the Botanical Gardens were also busy protecting the plants from the drought. In an interview, Cornelia Löhne, scientific director of the Botanical Gardens, explains the consequences of the heat wave for native plants and gives tips to gardeners.
How have the plants in the Botanical Gardens survived the heat wave?
Cornelia Löhne: The problem is not the heat but the drought. When you walk through the gardens you can see that the water level in the pond is more than half a metre below its normal level. Planting on the banks has also suffered because of the low water level. The lawns are completely scorched and many trees are losing their leaves.
Have there been comparable periods of heat?
Löhne: 2003 was similar. However, it seems to be even worse this year.
How have you protected the plants from drought?
Löhne: Above all, we have watered those plants that are important to us – to the extent it was possible for us to do so. Over the last weeks all our gardeners were busy watering the plants daily.
What protection measures are there apart from sufficient watering?
Löhne: Providing shade is important. We have placed delicate plants grown in pots or tubs and able to be moved into the shade. This protects them from excessive sunlight and overheating. This is especially important for young plants that have recently been cultivated.
Which plants have been particularly affected?
Löhne: Above all, shrubs and trees only planted in recent years. Some of them will probably not survive. Many trees can tolerate drought to a certain point. However, we can see that some have suffered severe damage. With others, you can first see the extent of the damage later when the weather changes. The tree often seems healthy on the outside, but the roots have been so badly affected by the heat that the tree can quickly become infected with a fungal disease after it rains.
How do gardeners know whether their plants and trees have been damaged?
Löhne: It is particularly important to observe trees and shrubs. A shrub that currently looks completely dried out must not necessarily be abandoned. In such cases, you should wait until next year. However, you should never trim hedges during hot periods. It is better to first leave the stressed plants alone and then cut them back hard after several rainfalls in the autumn. Dead branches on trees, which could fall down, are important indicators. However, such signs cannot be generalised.
What should owners of balconies and terraces bear in mind?
Löhne: Here too, it is important to provide shade. If you can, open your awning or parasol during the day. This protects against direct scorching. The shade reduces the temperature and therefore also evaporation.
How can gardeners protect their plants from heat and drought?
Löhne: It is important to continue watering. For those owning gardens, there are irrigation hoses available with holes in several places. This special hose can then be laid along a hedge or by a newly planted shrub. Water gradually drips into the ground. This has the advantage that not much water evaporates. With sprinklers or lawn sprinklers, a lot of water evaporates before it reaches the ground.
Should we therefore avoid lawn sprinklers?
Löhne: In private gardens, it is advisable to put lawn sprinklers on at night rather than during the day as less evaporates. In the Botanical Gardens we only put the sprinklers on during the day when we can monitor them.
What other garden tips do you have?
Löhne: If in doubt, garden owners should set priorities. Lawns can be restored faster than, for example, an old beech tree.
Heat waves will be more common in the future according to forecasts. Should we therefore completely rethink how we garden?
Löhne: Garden planners and civic planners are already looking into this and are trying to find alternative plants for our gardens. Plants that better cope with Mediterranean conditions and hot, dry summers. However, we cannot easily use plants from the Mediterranean here. In spite of climatic changes, the plants here must still be able to survive frost, which many Mediterranean plants do not. Thinking about new planting concepts will certainly be a challenge for the future. How we cope with climate conditions will also be a central theme of the annual conference of the Botanical Gardens Association that will take place in September in Bonn.
(Original text: Sabrina Bauer / Translation: kc)