Most gardeners know at least one Arum, probably Arum italicum. This charming woodland plant can be bought cheaply in garden centres and nurseries as either a bulb or a growing plant. For many years I was oblivious to the extraordinary variety of the genus; from small and cute to huge and chunky, foul stench to delicate aroma, Arum has it all! It is only a shame that many need the care of a greenhouse, but in a sheltered position it is possible that many many species may survive. Considering the rarity of many species it is best to bulk up your plants in the greenhouse and only try them outside if you have spare tubers.
CULTIVATION On the whole Arums are quite easy to grow. The shade/part shade species need a moist but free draining soil- their biggest threat comes from drought. However most species are sun lovers and are grown on what is known as a 'Mediterranean Cycle', that is a warm and dry summer and a cool and wet winter. Rather helpfully that's exactly what most of use get in the UK! The most important thing with this latter group is drainage; drainage should be very sharp as they will not tolerate sitting in water AT ALL.
Feeding is a good idea with Arums, especially if they are in pots! Feed with a high nitrogen liquid feed once a fortnight; this will feed the leaves which in turn will feed the tuber/rhizome.
Talking of rhizomes and pots... watch out for Arum concinnatum- although beautiful it is more on the tender side, but it also has an aggressive rhizome which, according to Peter Boyce's monograph on Arums, "travels a considerable distance each season, [and] makes conventional pots unsuitable". Presumably what he means by this is that pots need to be very wide to accomadate the rhizome, but also very deep to accomadate the root system. Needless to say mine was potted up and growing before I read this section of the Arum monograph; I may well go to the greenhouse one morning to find that Arum concinnatum has escaped from it's pot and is moving into pots with other bulbs!
This example does illustrate how Arums behave- a well fed Arum will bulk up in it's pot quite quickly, so use the following rule of thumb (useful for other aroids too): When potting up tuberous aroids, use a pot three times the diameter of the tuber. This has proved to be a very important rule- in my first year of growing some species they outgrew their original pots! They are now growing away quite happily in pots which I had originally thought were too big.
THE SPECIES (any marked with a * can be seen in the aroid gallery)
Arum pictum- I'll start with this because it is the first to flower. Firstly, I am talking here about Arum pictum, not Arum italicum 'Pictum'. The difference? Arum pictum is a large, autumn flowering, redish spathed species for full sun; Arum italicum is a smaller, spring flowering, green/white spathed species, and the form 'Pictum' has very attractively marbled leaves. If your Arum flowers in autumn it is Arum pictum- pictum is the ONLY autumn flowering species. To me this makes it much easier to identify!!
This Arum will grow outside in a sheltered spot in the garden, but hard frost will damage the leaves. Apparently it is a good idea to keep this Arum in a spot which does not get early morning sun (which warms the foliage too quickly, causing damage).
Arum apulum- I love this Italian species for it's beautiful spathe colour, a warm and spicy red/brown. So far it is proving to be quite easy in the greenhouse, and is neither slow to bulk up or invasive. According to the the monograph (Arums; Boyce 1993) A. apulum is found in south eastern Italy at altitudes between 300-400m along the 'heel' of the country. The spathe should be around 5-7" (12-18cm) long (making it a reletively small species when compared to others), and the leaves are smallish, making this probably one of the best species for pot culture.