Since she is now eating solid foods regularly, she really needs to be drinking water regularly. This is probably the root of her constipation problem. So long as she was only getting breastmilk, it wasn't necessary to supplement with water, but now that she is eating solids, water is absolutely essential - as it is for all of the rest of us who eat solid foods! At 6 months, she is old enough to start trying to use a sippy cup with water in it. She will need help at first, naturally, but if you get a sippy cup with handles, she will start to begin to try to use it on her own. You can also try "feeding" her water on a baby spoon.
In any case, she MUST start drinking water; this will help relieve her constipation more than any other remedy, herbal or not. Water is an essential nutrient, and we tend to be in bad shape if we don't get enough of it (especially in summer, when it is hotter).
In the meantime, the remedy you suggest is worth a try, in small dilute dosages. If nothing else, it is mostly water (!), so you may start seeing results. But she still will need plain un-sweetened unflavored drinking water in diet from now on. Hold off on juices; they tend to make constipation worse, and do not hydrate nearly as well as regular ol' water.