## 1 Introduction  
The city of Lugo (Galicia, Northwest Spain) is renowned for its third century Roman wall (Figure 1). The wall represents one of the finest surviving examples of late Roman military identifications and was declared a World Heritage site by UNESCO in 2000 (for further information, see references [1, 2]). In relation to this recognition, efforts have been made in the last 20 years to conserve the construction materials (schist and to a lesser extent granite, bound together with mortar), which are subjected to strong biodegradation phenomena caused by growth of biodeteroigenic vascular plant species [3]. Biodeteroiration affects two-thirds of the world's historical buildings and stone monuments [4], with most of the studies focused on microphyte populations composed of cyanobacteria, algae and lichens (see [5, 6], for recent reviews) and few focusing on the role of higher plants [4, 7]. The propensity of a wall to provide a habitat for biological colonization, i.e., the bioreceptivity of the wall in a broad sense, considering intrinsic and extrinsic factors [8], mainly depends on its size, the construction materials, inclination, exposure and age [9]. The establishment of vascular plants generally requires crevices, fractures and interstices in the wall [10], and the heterogeneity of the building materials, exposure and slope are key factors at this stage [11]. Weed control in monumental walls is of particular importance, because plants can damage monuments with their roots, give the appearance of neglect, obstruct access to sites by visitors and/or conceal the monuments themselves [12]. On walls, the damage caused by root systems of herbaceous species, which tend to spread as pioneers, can have a significant influence on biodeterioration, causing cracking, deformation and detachment [13; 14; 15]. In the Roman wall of Lugo, the weeds are removed annually, and herbicide treatments are injected in the roots and stems of vascular plants or by spraying to prevent mechanical damage to the stone. The breakage and loss of cohesion of the structure, rather than aesthetic factors (about which opinions may differ), lead to systematic treatments being applied to weeds on the wall every year. Mowing and clearing procedures such as cutting are commonly used for weed control. However, these may be insufficient because perennial vegetation rapidly reinfests the same sites because their roots remain alive and regenerate as soon as the climatic and substratum conditions are favourable [16]. This leads to the use of herbicides, among which, nitro-organic compounds (amides, diazines, triazines, piridines and urea derivates) and organophosphorous compounds have been the most commonly used [17], up until the time the present study was carried out. The ideal characteristics of herbicides include low toxicity, low risk of environmental pollution, high efficacy against biodeteriogens, wide range of action, easy to use, no interference with the substratum and low cost [18; 19]. In this respect, a low usable dose of herbicide, which can be encouraged by the method of application (for instance, injection in roots instead of spraying), generally entails an actual decrease in the risk to human health and the environment, along with less interference with the substrate [17]. Herbicides can be divided into two broad categories: contact herbicides and systemic herbicides. The former act in the proximity of the areas of entry, while the latter (which are the most commonly used) act after penetration and are transported to other parts of the plant where they act [17]. Historically, glyphosate-based herbicides have been widely used for weed control [17]. In the Roman wall of Lugo, in the late 1990s, chemical herbicides, such as glyphosate, were applied prior to further clearing by manual moving.