The Top 10 ‘missing' plants are:
To become a National Collection holder you need to grow a range of plants of the genus concerned - about 75% of your chosen plant listed in the RHS Plant Finder.
A representative for Plant Heritage said: "This may not be as hard as it sounds. There are only six types listed for Fatsia, 30 types of Nicotiana, and 37 for Alcea."
"It's not only big gardens or heritage properties that can house a National Collection. In a back garden in Harpenden, Hertfordshire, just 70ft long by 35ft wide, there are three National Collections - Thymus, Satureja and Hyssopus, fitted around many other plants and designed into the garden's style."
Collections are held by botanic gardens, universities, commercial nurseries as well as by private individuals.
Support is available for anyone starting a collection from the National Office and through a system of collection coordinators.
Contact Plant Heritage on 01483 447540 or www.plantheritage.com.