The problematic coexistence of the botanical features of the Oppian Hill and the underground archaeological structures of Trajan’s Baths and the Domus Aurea is well known.
The plants on the Trajanic Terrace date to the arrangement designed in several stages by A. Munoz in the 1930s and include:
Ulmus campestris (elms) facing the Colosseum, rows of Pinus pinea (umbrella pines), Quercus ilex (holm oaks) and Cupressus sempervirens (cypresses) flanking the wide avenue which cuts the Trajanic bath complex in two, completely ruining the visual unity of the archaeological system.
The area’s vegetation is currently in an extremely poor condition and some of the trees present pathogenic alterations with the consequent risk of problems to the plant or the detachment of dead branches. It was therefore essential to monitor the situation in detail. The botanical recording of 2011 showed the significant changes that had taken place over a period of six years and allowed us to identify interventions that could no longer be put off for the safety of visitors to the park, the trees and the monument. The 2012 update shows the most recent interventions.
The layer of earth weighing on the archaeological structures beneath
is on average 1.70 m thick: this entails a significant weight but is at the same time insufficiently thick to avoid the problems caused by the presence of the roots of the trees planted on the terrace. The older trees, which have reached their maximum aerial growth and root system expansion are extremely dangerous given the weight of the plant biomass, an added weight of several hundredweight only partially distributed by the roots.
These penetrate into the structures, creating cracks or enlarging those already present, in part due to the chemical action of the root exudates which can dissolve mortars. Exudates – chemical compounds released by roots especially in proximity to the root tips and older parts – react with mortar, causing it to dissolve and disintegrate, a situation worsened by the inevitable erosion by atmospheric agents. Over the years, in a slow but inexorable process, the mortar holding the monument together is lost, and thus the monument itself as well.
One of the first interventions carried out, in collaboration with the Rome Ufficio Giardini (Parks Authority), was to fell an Ulmus campestris (elm) affected by Dutch elm disease, a disease caused by a fungal parasite that was compromising its stability.
On the wall faces and tops, spontaneous plants become weeds.
In the West Wing of the Domus Aurea, above the rooms running north-south and forming the edge of the building complex – the so-called Little Barracks” – the presence of invasive plants was particularly serious. This made it necessary to carry out a mechanical and chemical weeding intervention.
We can speak of weeds in accordance with the definition
“objectionable plants in the sense of plant species growing where they are not desired”
Terminology Committee of the Weed Science Society of America
This is a particularly appropriate definition of plants in archaeological areas where, if not adequately controlled, weeds may seriously compromise the delicate equilibrium and create situations of conflict: between the need to safeguard the most significant features of the flora
from an environmental point of view or that of their historical significance, and the need to conserve unique archaeological structures, on which the flora has a detrimental effect. In these contexts, plants that in other cases might be important landscape features become “weeds”.
Mechanical removal (mowing) is neither fully practicable given the impossibility of extracting roots from walls nor effective since, with rare exceptions, regrowth is fairly rapid.
The cutting operation itself may be damaging and should be carried out with the necessary precautions to avoid causing trauma due to tears. Chemical interventions present other difficulties: the need to use effective products that do not interfere with the lithotypes making up the archaeological mortars, that are safe for operators to use and that do not cause environmental pollution.