sawdust
Those wishing to have organic material at the bottom of an elevated garden may consider wood chip mulch. While they don't provide the same drainage as gravel, they can be easily added in a way that still provides good water flow, and over time they break down, replenishing the native soil below. However, you should try to avoid pressure-treated lumber because the chemicals can still leach into the soil. So make a thick layer of organic matter, put wood chips on the bottom, and then a layer of grass clippings, hay, and leaves.
gravel or river stone
Other popular choices for the bottom of metal raised garden beds are pea gravel or river rocks. It provides drainage, allowing excess water to drain away from the plant's roots. Since it doesn't break down, it's more durable than other materials that are more prone to decay over time. However, it may be more expensive.
pine needles
Pine needles have many benefits. They are a viable option for the bottom of metal raised garden beds because they tend to retain moisture and decompose slowly, which means they improve the soil of the garden bed over time. Pine needles also provide a natural mulch that can help your garden retain moisture when it's hot and dry outside.
fired
This might be a good alternative to using landscape fabric. This probably won't last much longer than the latter, but it can stay under raised garden beds before fully decomposing. Because of the gaps between the threads, it allows water to pass through and provides better drainage for your metal raised garden beds. It is made from jute plant fibers so it is eco-friendly.
Geotextiles or landscape fabrics
Another popular option is landscape fabric, especially for those looking to minimize the time spent maintaining garden beds. Available at most home improvement stores, this fabric has good drainage and will keep weeds from growing from the bottom of your metal raised garden bed. Landscape fabric is not as durable as gravel or river rocks, but it is relatively inexpensive.
