Mulching your borders

As mulch is a clean material free from weeds that covers your existing flower beds stopping light from getting to existing seeds and preventing them from germinating. In the case of spent mushroom compost, it is broken down over a period adding lime and organic matter to the soil.

Mulch is a clean weed-free material that prevents annual weeds and improves soil quality
Your soil lacks organic matter which is important to hold nutrients and water in the soil. Plus as your soil is slightly acid adding a dressing of garden lime would be helpful. All these requirements can be satisfied by adding a mulch on top of the soil in the flower beds around the plants using spent mushroom compost about 50mm deep