Rudolf Weiss, the German herbalist, has some interesting ideas on low blood pressure. He describes it as being simply an aspect of a more deep seated problem. He also believes that when people have low blood pressure but no symptoms, that it does not require treatment. I do not suggest that this should replace your doctor's advice though! But I find it interesting, as a possible complementary treatment option, to consider his view that low blood pressure is the product of a weak circulation. As such, gentle herbs that support circulation may be a useful adjunct to whatever other therapy is suggested.
He stresses the underlying factor of a regulatory imbalance. And he suggests herbs like rosemary, that have a tonic effect, would be a useful support. Particularly in times of mental or physical stress, or after surgery, or some other illness that may have weakened the body, like the flu. He does say that treatment should only be for a limited time. He makes the point that life insurance statistics indicate that people with hypotension on average live longer than people with blood pressure in the normal range. (Weiss, p185)
Rosemary is quite a diverse herb. It is used effectively for headaches, particularly migraine. The herbalist Michael Tierra writes that many headaches are caused by stomach problems, and that the volatile oils in rosemary have a very beneficial and calming effect on the stomach. Rosemary also contains salicylic acid, which is the precursor of aspirin. Though being in plant form it doesn't have the long term side effects.
Rosemary is excellent for the circulation, and is often prescribed by herbalists for circulatory weakness (Fisher & Painter). The active constituents known as the flavonoids are heart and blood vessel tonics. So, as well as hypotension, rosemary is suggested for poor circulation, hypertensive headaches, and arrhythmias.
Externally, rosemary spirit is used for rheumatic problems (Weiss). It can also be massaged into the heart area.
Another active constituent of rosemary is rosmarinic acid, which is a phenolic acid. Rosmarinic acid has an anti-inflammatory effect. A study found that a rosmarinic preparation was effective in treating allergic asthma. It has also been found to suppress synovitis in mice. Synovitis is an inflammation of the lining of the joints, with swelling, which is quite painful. It is involved in rheumatoid arthritis. Rosmarinic acid can also prevent the activation of prostacyclin and complement, which leads to hypotension (Fisher & Painter).
Overall, this suggests that scientific research supports the herbalist's uses of rosemary for hypotension, poor circulation, and rheumatic problems such as rheumatoid arthritis. One problem has been that products which contained rosemary had quite low concentrations, although some supplement manufacturers have begun to make a very concentrated rosemary product. Concentrations of rosmarinic acid in these range from 5% to 70%. And given that rosmarinic acid is an incredibly powerful antioxidant as well, this would make it a good choice for preventing heart disease.