No, but it can make a very significant difference to the success of your perennial meadow. In the first year the greatest cause of failure is competition from young weeds including grasses. However well you have prepared your site the seed bank in the soil can still contain millions of weed seeds. Perennial meadow seeds germinate slowly over a period of time. On non mulched sites this allows the weed seeds in the soil to gain a competitive advantage quickly swamping the slower perennials. Mulching reduces this very significantly although it still doesn’t stop weed seeds from blowing in.