Objective The rapid development of global urbanization leads to the intensification of urban heat island effect, which poses a potential threat to human health and life safety. Roof greening can give full play to the ecological benefits of green space to alleviate urban heat island effect, but it faces some problems such as high maintenance and management cost and serious homogeneity of landscape. Spontaneous vegetation with high ecological value, low maintenance and vigorous spontaneous growth have the potential to become urban plantscape. Chengdu has the advantages of early development of roof greening, abundant species of wild plants, and support of roof greening policies. Therefore, this research took Chengdu as an example to discuss the climate adaptability of spontaneous vegetation in roof greening in summer, with a view to enriching types of roof greening plants at low cost and alleviating urban heat island effect.

Methods The habitat types of spontaneous vegetation in urban areas are determined by literature review, supplemented by data on 30 m fine land cover of Chengdu in 2020 provided by the geographic big data cloud platform (mainly referring to the distribution range of irrigated farmland to guide the collection of spontaneous vegetation). The classification and zoning of investigation for spontaneous vegetation is conducted through a comprehensive literature review and the application of big data analysis. The research contents include the variety types, morphological characteristics, root length and ornamental value of spontaneous vegetation. Finally, the collection of spontaneous vegetation and the experimental planting on the roof are carried out. According to the occurrence frequency, growth and reproduction of spontaneous vegetation identified during the investigation, 16 species of spontaneous vegetation are selected. Experimental verification method is used to construct a green roof module experiment platform with soil depth less than 10 cm which is on the sixth floor of a teaching building in Southwest Jiaotong University (Xipu Campus). Continuous adaptive observation was carried out during the period with the highest summer temperature from July to September, 2022, with the starting time of observation being determined as 9:00-10:00 a.m. to effectively avoid the strong exposure to sunlight at noon or afternoon that is likely to make plants soft and short and thus affect the observation data. The climate adaptive growth height of the spontaneous vegetation is recorded respectively at the beginning and the end of the experiment, and the ground coverage is recorded every 7 days. Such indicators as plant survival period, regeneration and spontaneous growth are continuously monitored during the experimental observation period. The observation data of growth height, ground coverage and survival rate of spontaneous vegetation in summer in roof greening are presented graphically using Origin to facilitate the comparison of results, and the regeneration and spontaneous growth of spontaneous vegetation are described in words.

Results According to the experimental results, the climate adaptation of various species of spontaneous vegetation to roof greening in summer is significantly different, and 10 species of spontaneous vegetation, such as Oplismenus undulatifolius, Cynodon dactyIon, Imperata cylindrica, Lactuca indica, Aster indicus, Mentha canadensis, Dichondra micrantha, Plantago asiatica and Acalypha australis grow well under the climate conditions of extreme high temperature and drought, and are thus deemed suitable for roof greening. Oenanthe javanica, Arthraxon hispidus, Setaria viridis, Lophatherum gracile, Dichrocephala integrifolia, Typhonium blumei and Solanum nigrum fail to adapt to the climate environment of roof greening, and are thus deemed unsuitable for roof greening in Chengdu City. In the process of adaptability experiment, it is found that high temperature, diseases and pests may have a negative impact on spontaneous vegetation. High temperature may cause the leaves of plants to turn yellow, curl or fall off, and diseases and pests may make the leaves mottled, curled, convex, etc., both of which may lead to plant death or poor ornamental effect. Rain and artificial irrigation can promote plant growth. Spontaneous vegetation such as Oenanthe javanica and Plantago asiatica will be revitalized after rain, enabling them to send out new shoots, and be less affected by pests and diseases. The vegetation species selected in this research can provide reference for not only the application of spontaneous vegetation in roof greening under the concept of low-maintenance plantscape, but also the introduction of spontaneous vegetation in the concept of new naturalistic ecological planting design.

Conclusion This research focuses on the climate adaptability of spontaneous vegetation in roof greening, which is seldom involved in the exploration on the application of spontaneous vegetation in urban landscape. Spontaneous vegetation plays an important role in the maintenance of landscape in urban bare and waste lands, which can effectively make up for the lack of ornamental value and ecological value of garden plants. From the perspective of building roof greening landscape at low cost and replacing single types of roof greening plants, spontaneous vegetation may serve as the key material for constructing pristine and wild low-maintenance landscape. The evaluation and screening of urban spontaneous vegetation with application potential may provide a basis for the selection of various plants that can be applied in urban roof greening, innovate the use of roof greening materials, and help to create sustainable low-maintenance plantscape.