About > The Propagation of Oaks

By James MacEwen, FLS, Fulham, London, UK

 

When possible, our oaks are propagated from seed collected from wild sources. The necessity of this is that they easily hybridise and rarely grow true from acorns collected from gardens. Even in wild-collected seed, natural hybrids may occur.

If the seed collected is healthy, acorns usually germinate easily, although we all, of course, have failures. At Chevithorne Barton, David Lancelles, the Head Gardener, grows the acorns in Rootrainers®. He uses a mix of three parts compost to one part grit. The compost used is multi-purpose with Sincro-Boast (a natural product derived from recycled organic materials). The grit is a coarse grit, washed, lime free and of horticultural grade. At the Sir Harold Hillier Gardens, Barry Clarke, who is the propagator and nursery manager, similarly works with Rootrainers®. He uses a peat-free soil mix made from compost bark, perlite for drainage and vermiculite for some moisture retention.

Acorns are placed on their sides and pressed into the soil so that 50% of the seed is above the surface. A fine layer of Cornish grit is then put on top of the seed: this allows for effective drainage but some moisture retention. Acorns are prone to rot if buried too deeply in sodden soil. Pests can be a problem and both squirrels and mice raid growing areas which are not secure. At Chevithorne Barton, a wire cover is placed over the Rootrainers®. Protection is equally needed against other pests such as slugs, snails etc. In cool conditions, acorns tend to germinate around February, March and April, although some species have been found to germinate in the second season – Lithocarpus edulis for example.

Other forms of propagation have been used successfully and certain species of evergreen oaks can be grown from cuttings. Q. acuta grows particularly easily but Q. semecarpifolia, Q. insignis and Q. rysophylla have also been rooted by Tom Hudson at Tregrehan. Cuttings can take a year to eighteen months to form roots. At Congrove, Christine Battle's best success has been achieved by taking semi-hardwood cuttings in early July. She places the cuttings around the edge of a shallow pot with four or five cuttings per pot. The cuttings are around 15-20 cm long and submerged by 2/3 of their length, their bases having been dipped in hormone rooting powder. All but three leaves are removed and, if large, about 2/3 of each remaining leaf is cut off. The pots are then watered, well sprayed with an anti-fungicide and placed inside a plastic bag, which has also been similarly sprayed. The bag is secured, placed on a heated bench and checked regularly to remove any cuttings that have rotted. As soon as there is sign of growth, the cuttings are removed from the heat bench. At the Sir Harold Hillier Gardens, the best method they found for rooting evergreen cuttings was by using mist with under-bench heating.

Oaks can also be grafted, and Barry Clarke has successfully grafted several Q. robur cultivars onto the species using both side-veneer and budding grafting techniques. Red oaks are grafted onto red oak stock and white oaks likewise on white oaks.

At Chevithorne Barton the young tress are transplanted from Rootrainers® into successive pots until they reach a height of 30-90 cm (1-3 ft). The young trees are very prone to attack by deer and badgers so suitable safeguards are needed. Tony Kirkham, Head of the Arboretum at Kew, is very insistent that trees should not be planted in round holes as the roots have a habit of following the edges of the dug holes and when doing this with a round hole, they form root balls, whereas if the roots find a corner they will force their way through it searching for fresh soil. The stem of the plant should be the same height as it was in the pot or ground that it came from on re-planting and great care should be taken to ensure it is not below this level. Introduction of mycorrhizal fungi to aid root growth is becoming more common and is certainly considered worthwhile. If the planting is in ground which has not had trees or shrubs on it recently, the mycorrhiza seems to stop some oaks from sulking when they are moved. Proprietary products containing mycorrhizal fungi such as Rootgrow™, which is in pellet form, are becoming widely available. Staking should be done as necessary.

In summary, oaks can be reproduced from seed, by grafting and, exceptionally, by taking cuttings. All three options have advantages and disadvantages. The disadvantage of germination is that one cannot be sure that the acorn has not hybridised. Grafting sometimes leads to trouble when the trees are fully grown in that the graft can be a weak spot prone to breaking at the join. Cuttings take with difficulty and those that do succeed tend to be from evergreen species.