Lettuce and bok choi going strong

First time gardener here. I planted some bok choi and lettuces 2 weeks ago.and last week in a sublet plot in a green house. One of the managers saw me planting and told me to find a way to keep the leaves from the soil, that some other gardeners use pieces of cardboard. I started with chopping and using some banana leaves,.but I didn't have enough to cover everything. Today I decided to use wood shavings as mulch and with the purpose to separate leaves from the soil, as they told me to. Let's see what happens!