Here at RHS Rosemoor we are currently in the process of replacing a large stand of sitka spruce in the Lower Woodland Walk (which are experiencing problems with spruce bark beetle), with a more ornamental planting, namely Sequoia sempervirens (coastal redwood) and Sequoiadendron giganteum (wellingtonia) - respectively the world's tallest and largest tree species!
We decided to grow on our seedlings using airpots, which are being used throughout the nursery trade for the production of trees and shrubs. The design of the pots encourages “air pruning” of the roots - which produces a plant with a root system ready to make its way into the soil and thus produce a strong healthy plant.
Initially, the seeds were stratified / scarified, firstly spending three to four weeks in the fridge and subsequently held under cold running water for 48 hours. After this they were sown in 3 litre pots and placed in a cold frame.
After two weeks the eagerly-awaited green shoots started to appear. The cold frame provided the necessary ventilation whilst providing shelter from spring rains.
As the seedlings neared the stage at which we would needed to prick out, the airpots arrived at Rosemoor. Firstly a thousand pots were constructed and then packed with a specially-prepared growing medium containing 75% bark-based compost. This time consuming task was made easier by the assistance of our two interns, Kieran and Paul, under the supervision of senior nursery horticulturist Catherine.
Investigations identified the optimum method of irrigation for the seedlings; immersion and then subsequent draining. Thus, as Catherine was busy pricking out, an immersion tank, complete with a clever flood and drain system was completed (credit to Phil of the Estates Services section for the design).
The seedlings have now been in the airpots for five weeks and despite a number of losses during the unusually hot West Country summer, the remainder seem to be thriving.
It has been an exciting project in which to be involved. Sequoiadendron giganteum are extremely long-lived trees, the oldest thought to be 3,500 years old. Who can imagine what the world will be like when one of these seedlings reaches that age!
Sequoiadendron giganteum (Wikipedia) (RGB Kew)
Sequoia sempervirens (Wikipedia)