28 gravel, pavers, dirt, or mulch, be sure to put landscape fabric underneath. This will save you time and hassle of pulling unwanted weeds on your greenhouse floor. If you are plagued with pests and/or disease, removing and replacing the soil can be a back breaking job. latest trend: biochar Newest Trend: Biochar has been used to manage weeds for many centuries and more recently, to improve soil conditions. Most commonly, the homeowner will use a sealed metal drum to burn organic matter such as grass, hay, leaves or wood. The matter is burned slowly and turned into a carbon rich soil additive that when combined with gravel or sand is very useful if you grow plants directly into the ground inside your greenhouse.