Zantedeschia
Common name: Calla Lily
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Overview:
The most common types of Zantedeschia are the half hardy aethiopica, elliottiana and rebmannii.
They should all be treated as half-hardy greenhouse plants although some forms of aethiopica can be grown outdoors in milder areas of the UK.
aethiopica (arum lily)
This is a deciduous bulbous half hardy perennials with dark green, slightly glossy, arrow-shaped leaves that originate from eastern and southern parts Africa.
The spathe like flowers can grow to a height of 500mm-1m (18"-39") and generally appear from March to June.
If planted outdoors they should be planted no less than 300mm (12") apart.
In the UK they should be treated as deciduous plants and dried off after the leaves have turned yellow.
elliottiana:
This is a half hardy perennial with speckled leaves that flower from May to June.
They quite often grow a little taller than aethiopica e.g. 600mm* (24") particularly if they are grown in deep shade.
When the leaves have turned yellow, store the rhizomes in their pots under the greenhouse staging.
They require minimum winter temperature of 8°- 10°C (45°-50°F)
rebmannii (pink arum):
This is a deciduous variety with lanceolate mid-green leaves that are quite often covered in silvery-white flecks.
The 3” long pale pink to wine-red spathes appear from April to June.
This variety is normally not as tall as aethiopica and elliottiana, where 300-400mm (12"-15") is more the norm.
Contact with sap of any of the varieties may irritate the skin, and any part of the plant may cause mild stomach upset if eaten!
Cultivation:
Aethiopica are generally grown in 150-250mm (6”-10”) pots of John Innes No2 potting compost (or similar), and require a minimum winter temperature of 7-10C (45-50F).
As mentioned above they can be grown outdoors providing the soil is humus rich, moist and in full sun.
In winter protect bulbs from frost with a deep mulch.
Week 13:
In Spring when growth appears, water moderately gradually increasing the amount until the plants are in full leaf, then water copiously.
Week 22:
Feed at weekly intervals from now until August with a balanced liquid fertiliser.
When flowering is over gradually withhold water, giving no water after mid July.
Week 38 - 42:
Fetch them back into the greenhouse and re-pot established plants.
Divide the rhizomes now and pot up the offsets then store them in their pots under the greenhouse staging.
Pests and Diseases:
Calla lily soft rot is the most common disease to affect bulbs.
This is caused by the erwinia caratovera bacterium.
Once infected, there is no chemical or other control method that will rid the tuber of this bacterium.
Good hygiene, air and soil temperature,and watering control is the best prevetative course of action to keep this disease at bay.
Thrips and Aphids:
To prevent the spread of viruses and fungal diseases spray the plants with a suitable insecticide as the flower spikes emerge.
Repeat this at 7-10 day intervals.
Temperature:
They require a minimum winter temperature of 4°-8°C (40°-45°F)
The ideal day/night temperature during the growing season is; 18°-24°C (65°-75°F) / 12°-18°C(55°-65°F) where the soil is around 18°C (65°F) but below 24°C (75°F)
Preventitive measures such as shading and mulches can be beneficial.
Compost / Soil:
They prefer loose, free draining soils with a pH of 6-6.5.
If drainage is a problem,place some grit under the bulb and into the surrounding soil/compost.
Alternatively, plant them into containers or raised beds.
If the soil becomes contaminated with the erwinia spores do not plant callas in that area for a number of years after.
Watering:
Plants should be kept just damp until the leaves unfurl.
Increase watering as the leaves open, and water regularly during the growing season to avoid the stress that may be caused if they become too dry.
Ease off with the watering after flowering and as the plant begins to die back.
Feeding:
Avoid high nitrogen fertilsers as this leads to soft growth that is more susceptible to disease.
Add a weekly balanced or high potash feed from the time the leaves begin to unfurl until flowering is over.