## 1 Introduction  
With the rapid development of highways, railways, bridges, hydropower, tunnel and mining fields, engineered slope failures are more likely to be triggered, leading to soil erosion, collapse, landslides and debris flow [1]. This seriously restricts the safety of the project and directly threatens the safety of people and property. Shallow landslides are a common natural geological disasters [2]. As an important ecological protection measure, vegetation has been used more and more [3; 4; 5]. Recently, mechanical functions such as shallow-root strengthening and deep-root anchoring were used to improve the erosion resistance and shear strength of the root-soil composite. Plant roots can effectively enhance the shear strength of slope soil, thus improving slope stability [6; 7; 8]. However, the mechanism of protecting a slope remains to be studied.  
At present, the stability of a vegetated slope is mainly studied through experimental research, theoretical analysis, numerical simulation and other methods [9]. The determination of the shear strength parameters of root-soil composites is of great importance, which is commonly conducted by remolding root-soil composites or testing in situ [10]. Comparably, the latter is more in line with natural conditions and is widely accepted by most researchers [11]. [PERSON] et al. [12] studied the influence of vegetation roots on soil shear strength by direct in situ shear testing. [PERSON] et al. [13] performed in situ shear tests to evaluate the effect of vegetation roots on the shear strength of soils. It is generally accepted that slope stability can be improved by the reinforcement effect of plant roots [14; 15].  
[PERSON] et al. [16] confirmed that vegetation could effectively reduce shallow landslides but had limited influence on deep landslides. [PERSON] et al. [17] studied the relationship between soil aggregate stability and shear strength under different dominant plant species through direct shear tests. [PERSON] et al. [18] carried out indoor straight shear and triaxial compression tests with different plants, which showed that the cohesion of the root-soil complex was significantly higher than that of plain soil. [PERSON] et al. [19] showed that the shear strength of soil increased with the increase in planting density. [PERSON] et al. [20] studied the influence of roots on the soil shear strength of collapsing walls by direct shear tests. On the other hand, numerical simulations have also been widely adopted to study vegetated slope stability due to their considerable advantages in terms of time, cost and repeatability. Many scholars have adopted the strength reduction method (SRM) such as [PERSON] et al. [21], [PERSON] et al. [22], and [PERSON] et al. [23]. However, the effectiveness of vegetation on slope stability is considerably affected by time. Namely, the different growth periods of plants affect the reinforcement of roots on the soil. Therefore, this paper considers the effects of different growth periods on plant root reinforcement.  
In this study, the shear strengths of root-soil composites during different growth periods and their effects on slope stability were investigated using laboratory experiments and finite element methods. The samples containing C.d and M.m roots were obtained from C.d and M.m planted on a slope, and the shear strengths during the different growth periods were measured. The effect of the plants on slope stability over time was investigated through numerical simulations.