You may (or may not) have noticed that my blog recently got a new name, Robbie’s rare plants. This naming of the blog has sparked a question in me: what is a rare plant?

The past month or so has seen me embark on a quest to find an answer. It has led me away from my day-to-day life, tucked up here in North Wales. It has taken me on a journey that, most of all, has reminded me that rare plants come in many different guises.

Whilst visiting RHS Wisley Plant Centre, I came across the Hawaiian cabbage palm, Brighamia insignis (aka cabbage on a stick). This plant is considered critically endangered by the IUCN and has a population of around 50 individuals in the wild, yet it is propagated in astounding numbers for commercial horticulture – 40,000 plants sold worldwide last year alone. You can buy a brighamia for just £15 at Wisley and many other large garden centres, nurseries and DIY stores all over the world. If you buy one, you are helping to fund the protection of the wild brighamia plants, which are clinging on to life on the edge of a cliff, being artificially pollinated by conservationists because their moth pollinator is now extinct. This plant is now directly dependent on humans for its existence.

At Kew, in a cage for its protection, a Wollemi pine, Wollemia nobilis, tells its story. It was only discovered in 1994 and numbers about 100 individuals in the wild. Threatened with extinction due to an infectious disease called phytophthora, its future in the wild looks pretty bleak. Yet in cultivation it prospers, with 4,000 plants sold in Europe last year. Some of the first plantings are now producing seed, which is being grown by horticulturalists in the UK. Certainly my Wollemi pine is doing well, putting on at least 2ft of growth each year. I wonder whether the Wollemi pine and the Hawaiian cabbage palm are still considered rare by horticulturists: are they the pandas and tigers of the plant world, imperilled in the wild, yet doing a good job of drawing in the crowds – and the money.

While chatting with the horticultural team at Kew, we discussed plants like Woronow’s snowdrop, Galanthus woronowii. Trade in wild-collected plants of this and all species of galanthus are monitored and covered by CITES appendix II, due to the potential of overharvesting. Kew is helping with research into the sustainable harvesting of G. woronowii for the horticultural and pharmaceutical industries. Because of this, wild plants remain common and I have no doubt about their safety. In gardens, G. woronowii is frequently grown, its wide glossy leaves contrasting those of its narrow leaved relative, the common snowdrop, G. nivalis. Earlier this year, however, a mutant bulb of it was deemed so “rare” that it was placed up for auction and achieved a record price of £725. Admittedly the plant in question was particularly unusual, but was it rare enough for the price it achieved? I doubt it will be long before that bulb is propagated in large numbers and made available to us all for a fraction of the price, probably with a plant patent, ensuring the auction buyer’s investment is returned many times over.

In my hanging basket last year I grew a plant that is virtually extinct in the wild, Lotus berthelotii (coral gem). I bought it at my local garden centre for £1.69. Surely it is rare enough to demand a price higher than that? Is the term rare not a little skewed when L. berthelotii is compared with the £725 galanthus bulb? And is the price we pay for plants a good measure of how rare they are? Perhaps only in cultivation.

A plant I love and grow at home is Richea pandanifolia or pandani, a relative of heather from Tasmania. The pandani’s tall, tree-like stems with mops of strappy leaves and white flowers create imposing silhouettes in the mountain forest home where it grows in profusion. It is not on the IUCN red list and is not threatened in its wild state, but in cultivation it is a rarity. When plants of R. pandanifolia come up for sale in the UK they are snapped up quickly. All species of richea are difficult to grow and propagate and hence the demand will always be greater than the supply. So: are Richeas rare?

Nymphaea thermarum, the world’s smallest water lily, is so rare that someone had the audacity to steal one from Kew. It is extinct in the wild and, at the time of the theft, was only held in two botanic gardens worldwide. This miniature treasure was the subject of a crime that stands up against Vincenzo Perugia’s 1911 theft of Leonardo da Vinci’s Mona Lisa. The joke now is that Kew horticulturist Carlos Magdalena knows the trick to breeding this water lily, and I doubt it will remain rare for very long. I feel it may end up being the next Wollemi pine or Hawaiian cabbage palm, paying its own way to habitat restoration and future reintroduction. I will certainly be first in the line to buy one, but will it then maintain its rare status?

It seems to me that the idea of a “rare plant” is as contrived as humanity makes it and the purpose that suits it; a product of economics. In horticulture, the next rarity is always around the corner. The new, the exciting, the next must-have piece of horticultural fashion will always be there. The Oxford English dictionary says it quite succinctly:

Rare: not found in large numbers and so of interest or value.

For me, however, rare plants come into a few types. There are those that are both threatened in the wild and of very little interest for commercial cultivation, overlooked by the world, the limelight hogged by tigers, Wollemi pines, pandas and Hawaiian cabbage palms. Then there are those that are of interest to cultivation but will never be produced and sold in large numbers for a myriad of different reasons. Finally, there are the old varieties of cultivated plants, the antiques of horticulture, pushed out of the way by the new and exciting, disappearing into the garden history books, whose genetics are so important to preserve the strength and vigour of the varieties that are pushing them out of the way.

The real rare plants are the ones whose genes will be lost for good unless we pay them some attention, and we really must pay them that attention.

• Robbie Blackhall-Miles is a modern day plant hunter’s propagator and gardener. He is personally interested in ancient families of plants and blogs about these on his website Fossil Plants. He also tweets as @fossilplants.