Does this leave a stale scent after using? I have used other plant based detergent that later leaves the clothes with a musty dirty oil smell

To get around the smell, you could try this trick. Get a wool ball and add a few drops of your favorite essential oil. Then toss the ball in the dryer in the place of a dryer sheet. The ball will scent the laundry and act as a reusable dryer sheet. If it ever looses its smell, add more oil.

Is this safe for grey water collection and garden reuse?

Be carful, my experience with using ECOS laundry detergent for grey water in my garden in California, the leaves of my raspberries and magnolia tree started to turn brown around the edges, so I stopped using ECOS for grey water. It may be okay for other plants, or in wetter climates, you may have to experiment a bit.

do you make a sent free laundry detergent

If you're wondering this because of being migraine-prone or otherwise unable to stand strong or unnatural scents—ECOS detergents are scented with natural oils and really do not leave a scent behind on clothes after drying. I can't take strong/synthetic smells and have been using them for a couple years. If it's about another issue though, I'm sure they do make scent-free, but it doesn't seem to be available on Amazon. At least not in the non-Pantry part.

Does anyone know if the "fragrance" listed in the ingredients is synthetic or natural?

I have been trying to get to the bottom of this as well. I think it depends on the scent. Lemongrass does not have “fragrance” listed, just two essential oils, and the lavender just has one essential oil. The magnolia lily scent has “fragrance” listed and when you go to their site, it just says a proprietary blend. In my experience, companies often hide behind “proprietary blends” to not have to disclose anything. I’m sure it is better than a lot of companies because they clearly state many bad things that are NOT in the detergents, however I do not believe the magnolia lily scent is just essential oils, or it would have them listed.