Much of the information written about Arisaema cultivation is scattered and not always relevant to growing these wonderful plants in the UK. It is hoped that these notes will tell you all you need to know for successful cultivation of Arisaema in the UK. So, if you have questions about outdoors Arisaema cultivation, read on!
Most Arisaema grow seasonally from an underground tuber, appearing above ground in the spring and dying back for the winter. Some tropical species are evergreen, or may grow from a rhizome. Despite their tropical appearance, many Arisaema take to outdoors cultivation in the UK easily, provided that a few basic rules are followed. An unhappy Arisaema is likely to slowly fade away over a few seasons, or simply die outright from the start. But by following these care instructions you are unlikely to go too far wrong and they should go on to give years of pleasure.
Planting Situation – it’s important to get this bit right!
Arisaema can be found growing from sea level right up to altitudes of over 4000m in the border zone between scrubby plants and true alpines in the Himalaya. So it isn’t surprising that they grow in a wide variety of habitats and that different species are exposed to different conditions in the wild. What is surprising, is just how well most species adapt to the rather cool and wet conditions of a typical British garden. But having said that, they are a little fussy in where they’ll grow and it is important to get their situation right.
One of the most important considerations is to protect Arisaema from harsh, drying sunlight. Arisaema that are planted in full sun will almost certainly end up with scorched, crispy leaves. This will cause it to prematurely die back, which in turn does not allow enough food reserves to build up in the tuber for decent growth the following year. Cool, dappled shade that is protected from the intense mid-day sun is perfect. Areas under the canopy of spreading trees are often ideal provided that the ground is not bone dry, packed full of tree roots and that the correct soil conditions are met (see below). Any area that is shaded, and that does not bake too hot and dry is a candidate for planting Arisaema.
Once you’ve found the correct site for your Arisaema, you’ll also need to make sure that the soil conditions are suitable. Get these two points right, and you’re pretty much there in giving your Arisaema the conditions they like. Correct soil type is extremely important for long term success. Arisaema tubers are very susceptible to rot if left in poorly drained soil, and once rot has set in, it quickly destroys the tuber. The soil should be moist, but free draining and porous (ie – plenty of air spaces within it). Incorporating leaf mould into the soil will go a long way to improving the soil structure. In the wild, some of the genuine woodland species are found growing in accumulated leaf mould and pine needles in the forest understory. It is often stated that by adding a layer of grit or sharp sand into the bottom of the planting hole, the drainage will be improved. Having experimented with this, I am not so sure that it works! Some growers state that the grit can cut into the tuber as it grows, causing damage which in turn leads to rotting. A mix of leaf mould and composted bark would perhaps be kinder to the tubers, but best of all is to thoroughly prepare the soil area as a whole before planting. If the soil is suitable there should be no need for further drainage. Certainly, do not use small grained sand, such as builders sand – this can compact together and form a ‘pan’ at the bottom of the hole which actually encourages water to collect. Despite being fussy with their drainage requirements, Arisaema are not choosy when it comes to soil pH. Either alkaline or Ericaceous soil is just fine although I would avoid extremes.
When grown out in the garden, it is recommended that Arisaema are planted relatively deep to avoid the risk of the tubers freezing during the winter. Species with large tubers (eg – Arisaema griffithii or Arisaema candidissimum) can be planted 4 – 6 inches deep. Smaller species will need to be planted at a shallower depth of 2.5 – 3inches (eg Arisaema jacquemontii). Arisaema tubers are best planted from mid to late Autumn through to Spring although it is most common to find them for sale from late winter onwards.
Arisaema can be greedy plants and respond well to extra feeding, especially at the beginning of their growing season when the leaves are unfurling. A long growing season combined with plenty of available nutrients will cause the tuber to bulk up faster, resulting in more reliable flowering. A granular feed, such as dried chicken pellets sprinkled around the plant and lightly worked into the soil gives good results, as does a dilute solution of liquid seaweed extract watered into the soil or used as a foliar feed. Do not fertilise when the plants are dormant.
If the soil conditions and the planting situation are correct, the soil should retain its moisture and not dry out too much in the shade. As such, Arisaema shouldn’t require too much extra watering, except perhaps during the height of summer or in very hot periods. Plants that are water stressed tend to make it pretty obvious: wilting leaves, crispy edges to the foliage or premature Autumn colour are all signs that the plants may need a drink. Water is best given in the evening using a hose with a sprinkle setting, or a watering can with a rose.
One commonly grown exception to this rule is Arisaema triphyllum. This grows in ditches and swampy areas in the wild, and does far better with regular watering during its active growing season. Plants that are well watered are quick to bulk up and form a clump. Arisaema will not need watering during the dormant period.
Pests and Diseases
When it comes to pests and diseases, Arisaema are usually trouble free plants. Grazers such as rabbits or deer seem to leave them alone, although slugs and snails can be a nuisance, especially with young plants and seedlings. Sap-sucking aphids are sometimes a pest on seedlings or young leaves that are just unfolding, causing misshapen or distorted leaves. It is best to remove these as soon as they are spotted. As already mentioned, tuber rot can be a problem. This needs to be treated as soon as it is found. Unfortunately, the first sign of a problem is often the collapse of the above-ground parts, and by this stage it may be too late. If rot is found, cut away the infected area of the tuber with a sharp knife until you reach clean, healthy tissue. Dab the area with kitchen towel to dry it off, then treat the wound with a dilute Hydrogen Peroxide solution. This helps disinfect the wound. Dust the cut area with yellow sulphur powder and leave to dry. Provided that the rot has been removed and controlled, the cut area should callous over again in time. Hydrogen peroxide is available from chemists and pharmacies, and usually sold as a 6% concentration. This dilution is fine to use straight from the bottle, but stronger solutions will need diluting down
Deciduous Arisaema don’t have many overwintering requirements. Obviously, only the below ground parts remain once the foliage has died back and so it is this that needs protecting from the cold. As already mentioned, the best advice is to plant the tuber fairly deep. A generous mulch of composted leaf mould will help insulate the tubers as well as feed the soil for the next year. Species that start into growth during late winter or very early spring can expose their delicate new growth to frosts. Covering with horticultural fleece or a cloche during the coldest nights is usually enough to prevent damage. A mild overnight frost is unlikely to be a problem but an unexpected late heavy frost can nip the leaves. Remember that an Arisaema will only produce one main shoot and the associated leaves and spathe each year, so to let it get damaged early in the season can leave an unsightly plant.