Oils are the classic format, but a leave-in serum is easier to live with if you wash often or dislike the feel of oil on your scalp. Nécessaire Rosemary Serum is the most concentrated here, with a 70 percent rosemary blend and five percent Capixyl, and it goes on as a light leave-in. BondiBoost makes two, the HG Lock Drops Daily Hair Serum and the Intensive Daily Scalp Spray, both made for thinning hair. Dr. Groot Roll-On Hair Thickening Scalp Serum has a cooling metal tip and menthol, which makes it the one I reach for after a workout.

Agent Nateur holi (growth) 84 is named for its timeline: the brand says to expect visibly thicker, fuller hair in 84 days. Crown Affair The Overnight Repair Serum works on the lengths while you sleep, with flaxseed and chia. Dr. Groot Pre-Shampoo Exfoliating Detox Water clears the product buildup that can sit on top of a scalp serum, and St. Meto Mother Clay Hair Restoration Treatment uses calcium bentonite clay with rosemary and cedarwood oils to lift residue.

How to apply a scalp serum: part dry hair into rows, put the dropper or roller directly on the skin, then massage for a full minute.

What I would skip: switching products every two weeks. The 2015 study saw no significant change at three months, so give one routine at least that long before you judge it.

Wash day is where rosemary is easiest to keep up with, since you are already standing there. Innersense has the story I like most in this row. Greg and Joanne Starkman had long careers in the beauty industry and had seen how many harmful chemicals went into salon products, and the birth of their daughter, who was diagnosed with Williams syndrome, made building a clean brand feel more urgent. They launched Innersense in 2005 and later chose to go direct to salons. The Hair Renew Scalp Hairbath uses salicylate-rich fermented willow bark to help with flaking plus green coffee bean extract, and the Hair Renew Soothing Conditioner is its match.

Nécessaire Rosemary Shampoo pairs plant proteins and ceramides with a microdose of Capixyl, and the Rosemary Conditioner is a lightweight multi-protein formula that leaves no residue. BondiBoost HG Shampoo is vegan and built on aloe vera, and the HG Miracle Hair Mask adds argan, jojoba and castor oils without silicones. RŌZ Foundation Mask is a deep conditioner in a seafoam tube, and Dr. Groot Hair Thickening Scalp Conditioner uses biotin, encapsulated prebiotics and rosemary.

A routine that works: pre-wash oil from the first row, then shampoo twice, then conditioner on the lengths.

What I would skip: a heavy mask on the scalp itself. Keep masks from the ears down and let the scalp products do their work up top.

Pick one rosemary hair oil or serum and use it on a schedule, a few times a week, for at least three months. Take a photo of your part in the same light on day one and again at month three, because day-to-day changes are too small to see. Massage matters too, so take a minute working the product in with your fingertips rather than just dripping it on. If your scalp gets itchy or red, stop, and see a dermatologist if shedding is sudden or patchy. Rosemary is a gentle habit, so pair it with good sleep and a steady routine.