We put a half-inch layer of fresh compost from Net Zero (they lease space on John’s farm) shortly after planting. These are the porcelain beds on a frosty December morning. The warm compost created a steam cloud on the frozen soil. The compost is weed free and lasts several months before the wind and rain wash it away of break it down.
We add another layer in late March to early April as the last feeding before the plants start forming scapes in May. We use compost as a mulch and a food source and it does help hold some weeds down.
Compost mulch also improves the tilth or composition of the soil and helps break down clay patches. This soil is like chocolate cake to work in. It crumbles easily, the worm feed on the compost and stay inside the raised beds adding their natural fertilizer to the soil.
Compost (or any similar mulch) makes weeding and harvesting much easier as well.
In our wet Zone 8 climate it is vital to plant your garlic in raised beds. Garlic is sensitive to having their roots in water so your soil must drain well. Sandy soil is fine but is lacks organics and without compost or other natural fertilizers the garlic size will suffer.
Our soil is black loam with some clay in patches. Field drainage is crucial and raised beds keep our plants out of sitting water. Even a few days of saturated soil will reduce your garlic harvest.