If viability of a stored species falls to 85% of the initial viability, seed may be grown out to harvest new seed, in a process called 'regeneration'. Where a species in Kew’s Millennium Seed Bank (MSB) is poorly represented or has very low seed numbers, seed may be grown out to harvest more seed, known as ‘multiplication’. This process is usually used for species that are rare UK natives, but may also be applied to overseas species such as Cylindrophyllum hallii from South Africa which is critically endangered in the wild. Plants are being grown in order to increase seed numbers. The plant material will be distributed to other ex situ conservation organisations (if the Access and Benefit Sharing Agreement permits this). The process of growing out leads to the development of propagation protocols which are vital for effective re-introduction and restoration projects.
Conservation collections held at the MSB have been used for several endangered species re-introduction projects in the UK. For example, Damasonium alisma (starfruit), Corrigiola littoralis (strapwort) and Apium repens (creeping marshwort). This work has primarily been in support of Natural England's Species Recovery Programme. The MSB Partnership has also been involved in the successful propagation of plants of Silene tomentosa from Gibraltar, a species thought to be extinct in the wild. Plants were grown from seeds stored in the MSB and using the knowledge gained from this process, plants have now been successfully reintroduced to the Rock of Gibraltar.