The size of deck screw to use will depend on several factors, such as the thickness of the decking boards, the type of material you're using, and the load requirements of your specific project. As a general rule, the diameter of the screw should be at least equal to half the thickness of the board. For example, for a 32mm thick deck board, you would want a screw with a diameter of at least 16mm. This will help ensure that the screw is strong enough to hold the board firmly in place without splitting the wood. In terms of length, you'll need to consider the thickness of the deck board, the depth of the joist, and any additional layers of materials you may be using, such as flashing or ledger boards. As a general rule, the screw should be long enough to penetrate the deck board and the joist by at least 25-30mm to provide a secure hold. When selecting deck screws, it's important to choose screws made from high-quality materials, such as stainless steel or copper, which provide excellent corrosion resistance and durability in outdoor environments. You can also choose screws with different head styles and coatings to suit your specific project needs.

If it were me and I wanted a clean and minimalist look, I'd choose a bolt/screw with a square drive head, and paint it to match (before installing it). I have seen smooth caps to cover the heads for Ikea furniture fasteners, but they were made out of plastic and relied on forcing a pin on the underside of the plastic cap into the hole in the fastener head. They didn't always sit right, though. I think something like this is your best bet: Try Googling "square drive screw," or "square drive wafer head screw."

Not sure where the "best reply" feature came from but it's certainly not one of my favorites. It seems though that all it does is "duplicate" the reply and put a copy of it at the top, so I have just been ignoring them and reading through the rest of the thread which is still "in order." I haven't noticed any "read more" but on some of the long threads it only shows the most recent replies and there is a button at the top to read all the previous ones. That isn't so bothersome to me when there's threads of HUNDREDS of replies and sometimes can take forever to load, especially if there as a lot of pictures. I had to dip out of one of the ones in "native plants" because it takes FOREVER to load.

They could be concrete walls which would provide the fire proofing. eta: I can't tell how big/heavy your planters are but Command hooks now have some rated for a pretty high weight. That might work for you instead of trying to drill into concrete. Or think about a plant stand instead of the wall? Or these, maybe? Heavy duty hangers