Some Rutaceae

 

The Family Rutaceae is a large and diverse group of woody plants consisting of about 160 genera and in habit varying from sub-shrubs and prostrate shrubs up to medium sized trees. They are principally evergreen and usually produce sweetly scented flowers and have aromatic foliage - from both of which oils may be extracted (viz Bergamot and Neroli from Bitter Orange). Many are used in local medicines and there are several in which the leaves are chewed regularly to maintain ‘well being’ (‘Buchu’ in South Africa).

 

The Family is chiefly limited to warm temperate to tropical areas but a few genera have species which fringe into a temperate distribution.

 

Although the great majority of species do not present any particular problems for the seedling producer some of those from particular provenances in warm temperate areas do not respond conventionally – especially those from arid ecologies in Southern Africa and Australia. The perceived wisdom for most of these was that they exhibited a hard seed coat condition but recent critical assessments have accepted that in the greater proportion of examples this is not the case. What is clear is that, in general, the cues which promote germination are very imperfectly recognised and understood - hence germination protocols tend to provide erratic results at best.

 

There is some evidence to suggest that in many of the relevant species it is not prudent to collect the fruits and extract the seeds before the fruit is ripe as the embryo is relatively late to mature.

 

Citrus

Citrus is the archetypal genus of the Family. It is a large genus containing many species of economic importance – Kumquat to Bitter Orange through Grapefruit, Lemon and the Orange complex. All are evergreen and have sweetly scented, usually white, flowers. As a result of cultivation over a millennium or more there has been much selection and eventually hybridisation and species are often difficult to recognise. Most are only satisfactorily cultivated in warm temperate to sub-tropical climates. Some selections produce apomictic seed.

 

Seedling production is not usually constrained and provided that the seed is fresh and imbibes satisfactorily, germination is prolific, rapid and uniform at 22+˚C.

 

There are a number of fruiting variants which produce particular varieties from apomictic seeds.

 

Tetradium

 

The genus Tetradium will probably be more familiar to most growers as Euodia (even Evodia!) but that genus is now limited to a substantial number of tropical species.

 

Tetradium is now defined as a genus consisting of (+/-) nine species of deciduous, pinnate, shiny leaved trees (to 20m or so) – which are native to the temperate and warm temperate climates of Eastern Asia. They produce a profusion of panicles of small, white to grey-white flowers which are a good bee forage.

 

The fruits are produced in the autumn and are bright purple-red capsules which dehisce to expose the two, shiny black seeds. These remain attached and adhering to the fruit case for some time if not taken by predators. The fruits and seeds can be rapidly taken by birds and so collection should take place before this occurs and then slowly dried to cause the seed cases to split.

 

A germination protocol will consist of a twelve hour warm water soak, followed by a 28 day chill to overcome a potential shallow dormancy and so synchronise emergence; germination is then speediest at 20˚C.

 

Most species have a warm temperate distribution but the Korean T. daniellii (with black-red fruits) and the Chinese T. hupehensis (with red fruits) together with the Himalayan T. fraxinifolium fringe into colder conditions and can be cultivated in the UK.

 

Phellodendron

 

The Amur Cork Trees (Phelldendron) are a relatively small genus of deciduous trees which are found in Eastern and North Eastern Asia. The number of species varies (about ten) depending on whether specific or sub-specific rank is attributed. They are characterised by the corky bark and the aromatic, thick, leathery, pinnate leaves. The leaves turn shades of attractive yellow in the autumn. All the species are dioecious.

 

The yellow flowers are produced in spring and these are followed, on the female trees, by the aromatic, fleshy fruit – which is a black-skinned drupe. The seeds are extracted by maceration, marginal fermentation and then separation after rubbing in running water. The seeds are black.

 

To achieve germination the seeds should be imbibed in warm water for 24 hours: they have only a shallow physiological dormancy which is mitigated by a chill of 35 days at 3˚C. This treatment will synchronise and speed germination.

 

Germination should be prolific at 20˚C.

 

The most widely distributed species is P. amurense (the Amur Cork Tree); however in most modern taxonomic treatments the species - P. sacchalinense, P. japonicum, P. lavallei and P. wilsoni - are all accepted as distinct geographical variants of this species. From a horticultural view it would be prudent to maintain these provenances and designate seed sources accordingly. P. chinense from China and P. molle from Korea are also found in cultivation.

 

Skimmia

 

This small genus of four species, of evergreen, ornamental shrubs, emanates from the warm temperate regions of Asia – from the Himalayas through China to Japan. S. japonica and S. laureola are dioecious while S. reevesiana is self fertile.

 

The evergreen leaves are entire and are distinctly aromatic when crushed. The dense upright panicles of individually small, white or pink flowers are produced in spring and are highly scented.

 

The brightly coloured fruits (red or black) develop in the autumn and are eventually taken by birds when ripe. The fruit is a fleshy drupe from 6 to 12mm in diameter and contains one seed.

 

The seeds are extracted by maceration and marginal fermentation and the dross is then separated off in running water. They are then washed and rinsed clean.

 

The species from colder provenances require a chill of up to 63 days – the length depending on the particular provenance - to ensure a synchronised and speedy germination. Germination at 20˚C produces a satisfactory response.

 

Poncirus

 

The monotypic Poncirus trifoliata (sometimes now reabsorbed into Citrus by taxonomists) – from Northern China and Korea is a hardy citrus which is used as an ornamental in its own right. It is also often used as an understock for citrus in marginal climatic conditions and to provide various disease resistances. It develops into a largish, sprawling shrub. Various cultivars have been selected.

 

It is a semi-evergreen shrub which can defoliate completely in cold winter conditions - especially in the UK. The leaves are compound trifoliate. It is dramatically thorny - with thorns from three to five centimetres long.

 

The typically citrus type, scented, white flowers appear in the spring and are followed by the fruits in the autumn.

 

The fruits, which are produced, on mature well established plants in the South of England and in other favoured, South West niches, in ‘good’ summers. The fruits are like small oranges - about the size of a golf ball – and containing many seeds (c30 per fruit).

 

The seeds are extracted by cutting open the fruit, when it is ripe, and picking out the seeds. They should be washed clean and then sown. Germination at 20˚+C is prolific and emergence will be in 10 to 14 days depending on the exact temperature.

 

Choisya

 

This small genus of three or four evergreen shrubs is native to Central America. C. ternata is best known and C. dumosa is also grown. The aromatic, tough, leathery, dark green leaves are trifoliate to pinnate. The scented white flowers are produced in late spring.

 

The fruit - a leathery capsule, containing two to six segments, is produced in the autumn.

 

The black seeds will germinate readily after imbibition at temperatures of 20˚+C.

 

Warm Temperate Southern Hemisphere Genera

 

There are a number of genera in this Family which have species with distributions in warm temperate climates – especially in Southern Africa and Australia – some of which will succeed in mild niches in the South and South West of the British Isles. They include many ornamental, small trees, shrubs and sub-shrubs which will provide potential in the event of climate warming (as well as species with defined medicinal value):- Acradenia, Agathosma (the Buchu of Southern Africa), Boronia, Coleonema (inc. Diosma), Correa, Crowea, Eriostemon, Geijera, Philotheca and Zanthoxylum. Most of these genera are native to arid, scrub, heathland and fynbos communities and hence favour acid soils; they are virtually all evergreen.

 

Many of these species have fruits which dehisce explosively and thus collection of the fruits should occur as the very first begin to go ‘ballistic’. Collect at this stage and then keep in a paper bag to dry and allow them to dehisce naturally in captivity.

 

It is in this group that is found the problem issues with successful germination under nursery conditions – usually germination is erratic and unreliable. In the past these taxa have been regarded as having hard seed coats but recent evidence indicates that they are permeable and, in the vast majority of cases, a warm water soak for 72 hours will achieve imbibition. However once imbibition is achieved most species still exhibit a physiological dormancy and it is the elucidation of this condition which has proved elusive. The environmental cues which promote the elimination of the factor have not proved to be easily ascertained.

 

It is important to process fresh seed as viability tends to decline rapidly. The embryo is morphologically mature at dispersal but sustains a physiological control to further development. Germination has been achieved in some species and quite arbitrarily after:-

a) an initial period (in excess of 30 days) of dry, warm (20 to 25˚C) storage – an apparent ‘ripening’ (a physiological embryo maturation?) period – before imbibition;

b) an extensive period (in excess of 42 days) of leaching – either in running water or as a warm water soak which is decanted and replaced on a daily basis;

c) a soak in commercial ’smoke water’ - although a successful outcome is not necessarily restricted to species from fire ecology communities.

 

Zanthoxylum has proved to be particularly difficult to deal with as it does appear to have an impermeable seed coat initially.