This is a brief guide to Iochroma with an emphasis on growing them in the UK. There is increasing interest in Iochromas which are late spring-summer flowering shrubs. Although a couple of species have been grown the in UK for a century or more they have been unjustifiably neglected in gardening books many of which do not mention them at all. In those books that do mention them they are frequently misnamed.
Only six of the 24 or so Iochroma species are in general cultivation but between them, and their hybrids, they give a wide range of flower colours in shades of blue, purple, red, orange and white. The flowers do not smell but are nonetheless attractive to bees in the UK and to hummingbirds in the US.
Iochroma australe (sometimes sold as Acnistus australis).
Iochroma australe is one of the most misrepresented plants around. It is usually sold under the name Acnistus australis, which is a valid synonym, but the correct name currently is Iochroma australe. If you want really to impress your gardening friends you can tell them that the true name for this plant is Eriolarynx australe, because unlike true Iochroma the flowers have hairy throats. But that name change has not formally been published and the mission of the moment is to get eBayers to stick to Iochroma australe as the accepted name. It is generally accepted these days that there is but one variable species of Acnistus, A. arborescens, which is not very ornamental.
The other misrepresentation of this plant is that it comes from Australia. Thomson & Morgan perpetrated this myth which has been copied among others by Chiltern Seeds, both of whom should know better. The epithet "australis" means "southern". Australia is the southern continent and "australis" or "australe" means "having a southerly distribution" not "from Australia". Iochroma australe comes from Argentina and Bolivia and is the southernmost of the known Iochroma species.
Iochroma australe comes mainly in blue-fading lavender and white-flowered forms. There are purplish and pink forms around but these are currently very rare. It is very difficult to render flower colour accurately in a digital image so do not be surprised if the actual flower colour you get is significantly different from that illustrated. There is a blue-flowered cultivar of Iochroma australe called ‘Bill Evans’ the name being given to a plant growing at Nottingham University. The original plant no longer exists but cuttings were taken before its demise. Unfortunately it is difficult to distinguish ‘Bill Evans’ from "ordinary" seedlings of Iochroma australe. The white-flowered form is sometimes called ‘Andean Snow’.
There is variation in flower size in Iochroma australe and there is scope for selecting large-flowered types. However, there are as yet no generally recognised large-flowered forms available.
Iochroma australe is the most cold tolerant Iochroma and can be grown as a garden shrub in many parts of the UK. In milder gardens it can be grown as a free standing shrub but in colder gardens it benefits from the protection of a west-facing wall. If cut back by frost it will rapidly re-grow in spring and flowering is barely affected.
These images emphasise the difficulty in rendering flower colour accurately. Both are the same plant of Iochroma australe under different lighting conditions.
This species from Ecuador to Peru is one of the bigger Iochroma species and grows into a large shrub or small tree. It is in cultivation although hard to find. It is characterised by distinctive tufts of brownish hairs on young shoots and leaves and by long 5 – 7 cm deep blue to purple flowers with a very large bluish white or blue calyx. The plant does grow outdoors on Tresco but it really is best for the, ideally, large greenhouse or conservatory. It is possible to grow it in smaller spaces by hard pruning in late winter before growth resumes.
There is a cultivar called ‘Vlasta’s Surprise’ but since the name was given to a plant in the Royal Botanic Garden, Edinburgh it will not enter wider cultivation unless "liberated".
Iochroma calycinum at RBG Kew.
Iochroma cyaneum from Ecuador is one of the more variable Iochroma species and forms with white, pink or apricot flowers have been described. However, the forms most commonly met with are dark blue, light purple-blue and violet-purple and in the UK these are represented by the cultivars ‘John Miers’, ‘Karl Hartweg’ and ‘Trebah’. Interestingly, in the US a similar colour range is represented by the USDA recognised cultivars ‘Royal Blue’, ‘Sky King’ and ‘Indigo’.
In the UK Iochroma cyaneum is a conservatory or greenhouse plant or a plant for summer display and must be overwintered frost-free. The plant does grow outdoors on Tresco and in very mild gardens it may be viable as a wall shrub but in most cases it should be grown as a container plant for summer display and moved under protection in winter.
'John Miers' 'Karl Hartweg' 'Trebah'
This species from Colombia, Ecuador and northern Peru is not in general cultivation in the UK although it is more common in the US. It has deep red to orange red flowers and rather glossy leaves for an Iochroma. It is botanically very close to the next species, Iochroma gesnerioides. In the UK Iochroma fuchsioides is likely to be best as a conservatory or greenhouse plant.
Iochroma gesnerioides (sometimes sold as Iochroma coccineum).
Iochroma gesnerioides is from Colombia and Ecuador so its range overlaps with that of Iochroma fuchsioides to which it is closely related. It is not easy to distinguish Iochroma fuchsioides from Iochroma gesnerioides. Like Iochroma fuchsioides, Iochroma gesnerioides has essentially orange-red flowers. The form most commonly met with in the UK has been given the cultivar name "Coccineum" implying that the flowers are scarlet. The actual depth of flower colour achieved may be related to seasonal and cultivation conditions. A yellow-flowered form has been reported but this is probably no longer in cultivation.
Iochroma gesnerioides ‘Coccineum’ has the reputation for being trickier to grow than other Iochroma and subject to die back with plants re-sprouting from the base. In my experience, however, it is no harder to grow than other Iochroma, many of which are prone to die back in winter.
In the UK Iochroma gesnerioides is a conservatory or greenhouse plant or a plant for summer display and must be overwintered frost-free. The plant does grow outdoors on Tresco and in very mild gardens it may be viable as a wall shrub but in most cases it should be grown as a container plant for summer display and moved under protection in winter.
Iochroma grandiflorum is from Ecuador and Peru and potentially one of the larger Iochroma species. However it is easily grown and, through pruning, kept manageably small in a container. As the name implies the blue flowers are among the largest in the genus. The calyx, the cup-shaped structure at the base of the flower, is usually highly inflated.
The leaves of Iochroma grandiflorum are covered in viscid hairs and in humid conditions give off a peculiar smell rather like a wet dog.
In the UK Iochroma grandiflorum is a conservatory or greenhouse plant or a plant for summer display and must be overwintered frost-free. The plant does grow outdoors on Tresco and in very mild gardens it may be viable as a wall shrub but in most cases it should be grown as a container plant for summer display and moved under protection in winter.
Iochroma ‘Purple Haze’.
There are an increasing number of Iochroma hybrids becoming available, particularly in the US. ‘Purple Haze’ is the only hybrid likely to be met with in the UK. It is a cross between Iochroma australe and Iochroma cyaneum ‘Trebah’ bred by Julian Shaw in Nottingham. This is a vigorous free-flowering plant. It may be viable as a wall shrub but in most cases it should be grown as a container plant for summer display and moved under protection in winter. Because of its vigour it needs hard pruning in winter and is perhaps best grown as a standard.
As has been mentioned above, under most UK conditions, most of the Iochroma species are best grown as container plants. Any good quality potting compost is suitable although a John Innes type mix may be useful to add weight to stabilise containerised plants grown outdoors. They may be grown as greenhouse or conservatory subjects or stood outside in summer and returned to frost-free conditions in winter. They may be pruned hard in winter as necessary to maintain shape and size and are generally amenable to being trained as standards. Old plants may be rejuvenated in early spring (after winter pruning) by removing half the root ball and replanting in fresh compost in a similar sized container. 15 litre containers will be large enough in most circumstances although younger plants can be flowered successfully in 3 litre pots.
Water freely in growth but keep plants drier in winter although do not allow to dry out completely.
Feed weekly or fortnightly during the growing season with something like Vitax 2:1:4.
What to buy.
It is best to buy plants in compost despite the increased postage. If sellers are prepared to remove some of the compost to reduce shipping weight so much the better. Seed is sometimes available. Because Iochroma hybridise quite readily it is always possible that the seed will give plants that are not true-to-type. Seed is generally not a good way of buying true-to-type species but if the seller can offer reassurance as to controlled pollination so much the better. Under no circumstances is seed a good way to buy true-to-type hybrids although you may possibly get an interesting new plant from hybrid seed. A minor issue with seed is that it maybe contaminated with inert sclerosomes.
When to buy.
Both plants and seed can be safely bought all year round. Plants bought in winter will require frost protection. Seed stores well at room temperature for a few years and is best sown in early spring in a standard seed compost with gentle bottom heat.