Bare rock, uneven soil thickness, and drought have resulted in a harsh habitat in karst areas. Therefore, plant growth in these areas requires more advanced adaptation strategies. To explore the effect of unique soil habitats on herbaceous plant (Gramineae) growth under different water conditions in a karst area, we selected three container types[control (CK), deep (D), and shallow (S)] for six soil treatments in the following combinations:CK-CK, CK-S, CK-D, D-D, S-D, and S-S. Root growth, biomass accumulation, and root spatial distribution of Lolium perenne L. in different water levels (W₀, W₁, and W₂) were observed. The results showed 1) with sufficient water (W₀), the root length, diameter, surface area, and biomass of L. perenne in the compound containers that contained S or D were lower than that of the control (CK-CK). The degree of inhibition of L. perenne in treatment combined with shallow soil containers (S-S, S-D, CK-S) was greater than that in treatment combined with deep soil containers (CK-D, D-D). The root growth in treatments containing S (S-S, S-D, CK-S) were more inhibited than those in the treatments containing D (CK-D, D-D). With reduced water content (W₁ and W₂), root growth in the compound containers that contained D (D-D and/or CK-D) was significantly higher than that of the control, whereas it was significantly lower in the containers containing S (S-S/CK-S). 2) Comparing root growth in two sides of one compound container, we found that under W₀, both deep and shallow soil inhibited plant growth. Under W₁ and W₂, root growth of the S sides was inhibited severely. However, root growth occurred in the D sides. 3) Under W₀, the root-to-shoot ratio was not affected by soil space. With decrease in the water content, the root-to-shoot ratio increased in the D compound containers, whereas it decreased significantly in the S compound containers. Thus, the physical space limitation of different soil habitats affected plant root growth and biomass accumulation and distribution; however, the decrease of water content could change these responses:with sufficient water, soil physical space was the main factor affecting root growth and biomass accumulation and distribution. Correspondingly, L. perenne mainly developed shallow roots. However, when water was reduced, L. perenne roots could not obtain enough moisture in the shallow soil to supply metabolic growth activity. Therefore, L. perenne generally distributed limited organic matter to the roots and strengthened water absorption through root elongation, surface area and volume increase, and diameter thickening, to enhance drought resistance and improve adaptability to shallow soil and drought.