Q. I have been given a compost bin but my garden is all concrete. Can I use the bin successfully? PO, Gosport. A. Yes. Compost must be kept nice and moist and on a concrete base the material you want to turn into wonderful compost will dry out. Put a piece of thick carpet or several layers of thick cardboard under the container and this will keep the compost moist and will also stop stains on the concrete. I am really pleased you are going to produce compost. Well done to you. Q. Can you tell me the name of the weed which grows in the spaces between paving stones. The weedkillers I have tried do not work. MM, Fareham. A. Any weedkiller containing glyphosate will kill this weed but it takes two weeks to do the trick. Roundup is a very good one to use. It works well but the weeds return because the parent plants produce lots of seeds. You need to use Roundup three or four times a year and it will work really well. Q. I saw a box of zinnia seedlings at a charity event and bought them. The plants are five inches tall and they are lovely and green. But my mother-in-law tells me I have wasted my money. What do you think? MV, Waterlooville. A. Zinnias are beautiful if used as dot plants in a border. Plant single plants out in gaps using three inch each gaps and have the pleasure of showing your mother-in-law how beautiful they are in late September. Make sure you protect them from slugs.