Deadhead Verbascum?

(photo of Verbascum thapsus)

A woman called the other day to ask a question about her Verbascum. She has a number of Verbascum that are now in their third year in her garden. They have bloomed prolifically every year and she has been very happy with them. The question that she asked of me had to do with the flower spikes that have now passed bloom. Is it ok to deadhead Verbascum or is it better to leave the flower spike on the plant?

This is a great question, and there are different ways of answering it.

Aaron’s rod (Verbascum thapsus) is a plant I enjoy in the garden because of the large rosette of silver, felted leaves and the dramatic yellow flower spike. As I learned more about the plant, specifically its drought tolerance and ability to adapt to a wide range of soils, it only became more attractive in my mind. The one drawback however is that this Verbascum is considered to be a true biennial. A biennial is a plant that goes through its entire life cycle in two years. The first year Verbascum typically goes through a period of vegetative growth, growing that large rosette of foliage. It is then in its second year that the plant puts on its reproductive growth, the tall flower spike. Because Verbascum may die after flowering in its second year, I was taught to recommend leaving the flower spike on the plant to increase the chances of the plant self-seeding. The straight species of Verbascum thapsus are known to self-seed quite readily throughout the garden and seedlings can be transplanted to desired spots or easily removed if that is your wish. There is another species, Verbascum phoeniceum, commonly called purple mullein, and it too is considered to be a freely self-seeding biennial.

Verbascum hybrids, on the other hand, are quite a different plant. Once it was explained to me that this woman’s plants are blooming every year I knew that they were most likely not a straight species, but more likely some of the newer hybrids that growers are developing. For Verbascum hybrids, deadheading is preferred for a couple of reasons. For many hybrids removing the faded flower spike and preventing the seeds from being dispersed works to trick the plant into behaving more like a perennial and encourages it to come back year after year. The second reason to deadhead your hybrids is because regular deadheading helps promote the plant to produce new side shoots from the main parent plant. Next spring you may be able to carefully remove the small side shoots, or “pups”, as long as they have ample foliage and roots and transplant them to space where they can grow on their own. Therefore, to answer the question, if you know it to be a straight species, I would not deadhead it if you want it to seed itself in your garden. If you know your Verbascum to be a newer hybrid with a clever named cultivar (‘Helen Johnson’, ‘Raspberry Ripple’, etc.), then go ahead and deadhead it so that it produces new pups that you can hopefully then divide and transplant in the spring.