How can I stop weeds from growing in my interlock?

The primary step, after building it right in the first place, is to simply keep it clean.

Sweep it well, and maybe hose it down from time to time.

Do that often, and most of your problems will be dealt with.

Another good thing to do is to use a cleaner that discourages moss and weed growth. Spray that from time to time to supplement your brooming, and everything should be great!

If you find your interlock sand deteriorating, you can also add supplementary sand every couple of years.

Adding interlock sand keeps the joints from emptying, and we do not want them to empty, because then dirt will crawl in there and stick!

Build it right and keep it clean. No weeds will grow.

In case you were wondering, the interlock in the photo above was neither built right, nor kept clean.