The answer to 'why is my deck dying plants' is that the deck may be blocking sunlight and preventing proper soil drainage, leading to plant decline. Decks can cast shadows that limit the amount of sun reaching plants, and the raised construction can impede water flow, causing soil to stay saturated and stifle plant roots.
To troubleshoot this issue, first check if the deck is shading the plants for most of the day. You may need to move plants to a sunnier location or prune back any overhanging deck boards. Also inspect the soil around the plants - if it seems constantly wet, you may need to improve drainage by adding gravel or amending the soil. In some cases, it may be best to remove the plants from under the deck entirely and find a more suitable location for them.