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Sorbus seeds

The seed of a great idea

It’s not only animals that need help to survive – we work with a very curious tree. Whitebeam may not be one of our most familiar trees, but it’s both important and fascinating.

Whitebeam belongs to the genus Sorbus, and has distinctive white backed leaves and autumnal berries. It’s curious because it’s a tree that has a tendency to throw up new species, vexing scientists but giving a fascinating glimpse of evolution in action.

Paignton Zoo gardener Lorna Nicol-Griffith plays a key backroom role. Each year, fruits are collected, the seeds harvested and seedlings grown to create a nursey stock. It sounds simple – but it really isn’t.

The first stage of the process is to collect the seeds from the berries. Each berry has around 4 seeds, but the tree’s energy is normally focussed on just one of these, leaving the other seeds much smaller and flatter – these will be less viable for planting.

The larger seeds are rinsed and washed in deionised water. Lorna’s general rule of thumb is that if the seed sinks then it is more likely to grow than a seed that floats. But some seeds just need to absorb a bit of water, so they are given a few minutes.

It’s important that the seeds are as clean as possible and that there is no pulp left on them from the fruit to go mouldy.

The seeds are then carefully tipped out, separated (Lorna uses a porcupine needle for this) and dried on paper towels. She’s careful to avoid contact with skin as this could cause chemical breakdown.

To break the dormancy of the seeds, they need the cold – so, to simulate winter, the seeds are kept in a fridge for a few months. They won’t germinate until they are warm and planted in soil.

In March, the seeds will be taken out of the fridge and planted in compost. Lorna: “Within a month, we should see germination and by May, the seedlings will be showing.” The seedlings are checked to make sure they have a good root before pricking out and repotting them in larger pots.

Over the next year, the seedlings will have to be re-potted periodically to give them more space to grow. By the following summer, the saplings will be ready to move into 7.5 litre air pots. These are specialist pots covered with small spouts with holes that encourage the roots to grow outwards rather than tangling themselves up, creating a healthy root ball. When the trees are planted, they will have a strong healthy root system and can anchor themselves well. This process is repeated for each of the six species collected.

Cuttings have been taken from whitebeam trees to see if they would root. This would help to speed up reproduction and reduce the need for processing the seeds, offering another way to increase these tree populations quickly.

Lorna is passionate about horticulture. “People often don’t care enough about native species, they want exotic trees and plants which they feel are more unusual, but we need to preserve what we have to support and maintain our ecosystem. The loss of any tree, plant or species comes at a cost to our natural, beautiful landscape and the life it supports.”