Decks present an interesting conundrum when it comes to killing weeds. When the deck is high enough off the ground that you can walk under it, weed killing isn't much of a problem, but lower decks make it difficult to get to the weeds. The tiny spaces between the deck boards allow rainwater and just enough sunlight to touch the ground below that weeds can flourish, but you might not be able to physically reach the weeds to remove them. Using herbicides and ground covers can help rid the area of weeds.
Depending on the height of your deck, you might be able to reach the weeds to pull them out. If it's high enough to crawl or walk under, pull the weeds as they pop up. For lower decks that still have some space between the boards and the ground, use a long-handled hoe to chop down the weeds. This works well to kill annual weeds, but perennial weeds might grow back from the roots, which you might not be able to remove, given the limited space.
If water can get through the spaces between the boards on your deck, so can herbicides, even if you can't access the deck from the sides. Choose a ready-to-use, nonselective herbicide such as one containing glyphosate, then spray it between the boards to coat the weeds below. If any weed leaves or stems poke between the boards, coat them completely with the herbicide. Reapply the herbicide to weeds whenever you see weeds appear. When using the chemical, always wear protective clothing such as long sleeves, closed-toe shoes, gloves and safety goggles. Keep children and pets out of the area until the spray has dried.
Instead of constantly watching for weeds, cover the area under your deck to help smother the weeds and existing seeds. For a quick fix, spread a 2- to 3-inch layer of mulch under the deck. Organic mulch, such as wood chips, breaks down over time, but using a more permanent solution helps keep the area weed-free for longer. For example, spread black plastic or landscape fabric over the ground under your deck, securing the corners with landscape stakes. The black plastic keeps water and sunlight from touching the soil so weed seeds can't germinate, while landscape fabric keeps the weeds from growing while still allowing the water to drain into the soil.
Limiting the amount of water that gets under your deck helps keep the weeds in check. Instead of using sprinklers that spread water over a large area, use drip irrigation for the plants near your deck. This helps prevent weeds. For organic control that kills existing weeds and prevents most new weeds from growing, add 1 cup of salt to 2 cups of water and pour it over the ground under your deck. Using boiling water is effective at killing existing weeds. The water helps the salt soak into the ground, where it changes the soil's salinity to make it inhospitable for most plants.