Rest Grazing from the Critical Period of Soil Thawing in Alpine Meadow of Tibetan Plateau Is Conducive to the Sexual Reproduction of Polygonum viviparum

(1) Background: The most important management measure and utilization method for grassland is grazing. The suitable beginning period of spring rest grazing in the alpine meadow was selected to provide a theoretical basis for more scientific management and sustainable utilization. The experimental site is located in the cold-season pasture of an alpine meadow, which is located in the eastern part of the Qinghai Tibet Plateau. (2) Methods: We set up five treatments with the critical soil thawing period, the late soil thawing period, the grass reviving early period, the grass reviving late period with local traditional rest grazing period as the start time of rest grazing, and the grass withering period as the end time of rest grazing, recorded as P1, P2, P3, P4, and P5 (CK, control check). We studied the reproductive characteristics of the dominant plant, Polygonum viviparum, during different rest grazing periods. (3) Results: The results showed that, in general, P. viviparum was mainly involved in asexual reproduction and tended to have sexual reproduction with the rest of the grazing in advance. The biomass proportion of sexual reproduction organs and asexual reproduction organs in P1 were higher than those of other treatments, which were 704% and 20% higher than P5 (CK), respectively. The seed yields of P1 were 135%, 535%, 690%, and 1269% higher than P2–P5, respectively. The indices of P. viviparum seed quality in P5 were lower than those of the other treatments. The seed length, seed size, seed setting rate, and satiety grain weight ratio of P1 were higher than those of other treatments. We use the TOPSIS (Technique for Order Preference by Similarity to an Ideal Solution) analysis method to comprehensively evaluate the indicators of the five plots. (4) Conclusions: It was concluded that the rest of the grazing from the critical period of soil thawing was conducive to the growth and sexual (seed) reproduction of P. viviparum. Therefore, the resting grazing period of alpine meadows in spring should start with the critical period of soil thawing.