Deeper knowledge of how perennials work can help to save money and give clients a better understanding of long-term maintenance costs, a conference heard.
Speaking at the Palmstead Soft Landscaping Workshop in Ashford, Kent, earlier this month plant designer and writer Dr Noel Kingsbury said much of the nursery business is geared towards the retail market with perennials genetically programmed to be short-lived.
"Horticulture has been shockingly bad at collecting longer-term performance data," said Kingsbury. Designers need a new "rabbit's eye view" to truly understand how perennials will behave in the long-term, he added.
He advocated three categories: short-lived (non-clonal), potentially long-lived (clonal) and long-lived but slow to establish perennials (clonal), or as he put it at a conference in Ukraine, plants are either "sensible investors", "Vladimir Putins" or "Breshnevs".
Kingsbury revealed the results of a study by 66 retired gardeners who recorded the longevity of perennial plants, scoring them for longevity, spread, competition, speed of establishment and growth and to what extent they self-seeded.
The study found Echinacea purpurea died first with Miscanthus sinensis lasting the longest. Kingsbury showed how Echinacea, and particularly its hybrids, have one point of connection to its roots. Knautia macedonica is another example.
But others have better vegetative spread. Geraniums and helianthus have several independent root systems and are good at covering ground. These are not too rapid and have the ability to form a network of shoots.
"There's nothing wrong with short-lived perennials so long as you know they are short-lived," said Kingsbury. "When you have great blocks of something (in your design) and then you have something that starts to die, that's an embarrassment.
"Quite a lot of these plants have been heavily promoted by the retail sector and are not appropriate for the landscape sector. There are perennials that are genetically programmed to be short-lived and it is important to understand that."
However, good understanding of soil, conditions and surrounding plants is also crucial, he said. Some perennials such as Lysimachia punctata spread well but only fill in vacant space. The goal is a dense mat with a range of different species.
RHS Wisley curator Colin Crosbie, Queen Elizabeth Olympic Park project leader Phil Askew, Landscape Institute president Noel Farrer, Andrew Kauffman of Quadron Services, James Hewetson-Brown of Wildflower Turf and James MacPhail of Carbon Gold Biochar also spoke at the event.
Perennial plants - Kingsbury's categories
Guerillas or "Vladimir Putins" send out runners in an unpredictable way. "People are a bit frightened of them." Example: Sanguisorba armena.
Phalanx or "Breshnevs" march out steadily "with the tanks". Slow-growing ones form a circular clump, faster-growing ones are more uneven. Example: Kirengeshoma palmate.
"Sensible investors" send out roots and while it may take a long time to see results the plant is working on the long-term domination of the environment. Example: Baptisia australis. "Roots go down 20ft. It needs time to establish but once it is it is very long lived."