The tree dahlia, Dahlia imperialis, king of the dahlias

While many gardeners grow dahlias, few even know of the tree dahlia, Dahlia imperialis, the largest of them all. Though it can attain tree size, like other dahlias it is an herbaceous plant that dies down to the ground in cold weather. A native of Central America’s highlands, it nevertheless can be grown in fairly cold winter climates. Beyond the plant’s amazing size, the flowers are the main drawn in the garden, and they couldn’t come at a more novel time – just before the first frosts of winter hit.

D. imperialis is a large herbaceous plant standing typically between 2-6 meters tall, with extra vigorous specimens reported to top out at 9 meters. The fleshy stems grow singly or in clumps, starting out vertical, but eventually bend in a gentle arc by flowering time. They are segmented with nodes around 30 centimeters apart, giving them a superficial bamboo-like appearance. The leaves are compound in structure and are borne in opposing pairs. They look much an elderberry leaf (Sambucus sp.), with broad leaflets that are rich green in color. Underground thickened stems form clumps of elongate tubers with relatively few wiry roots.

The flower heads form rapidly late in the season, with flowering starting in early November. Each growth can hold multiple heads of flowers, bearing 2-20 pale purple flowers 10-15 cm across. Flower color is variable, ranging from mauve (most common) to pale pink and even pure white. The flower center is orange-yellow. Double flower forms also exist, both pure white and pink.

The tree dahlia is a plant of relatively high mountains, being found above 1500 meters elevation from Mexico’s central ranges, and southward to Guatemala and Panama (some sources report them from Columbia as well). These are plants of open, sunny environments growing in well drained, yet rich moist soils. In their native range some frost can be expected, hence their adaptation of going dormant in winter.

Growth begins relatively late, even here in southern Japan, usually not before mid May. In cooler maritime climates they may start even later. This is not a problem since they explode into growth during warm, wet weather. I’ve noticed that plants will grow up to around 2-3 meters tall by August and then they seem to slow down, becoming almost bushy at the top end. Then sometime in October they continue their rocket upward, growing another 2 or more meters higher, rapidly forming their flower heads which typically bloom around the second week of November.

One issue with this plant is its late flowering period. It seems to flower after most plants have died down and with winter just around the corner. If you live in an area where the first frosts don’t come before December, then flowering will commence normally. If however frosts hit in October or November, you’ll never see your plants flower, even if they grow normally and overwinter well. This seems to be a common complaint with plants being grown in the UK and the eastern USA in cold hardiness zones below zone 8, where early season frosts are common.

Of course in climates where winters are cool to warm, and frosts come sometime after December 1st, their late flower schedule is a bonus – a dahlia on steroids flowering when most plants are going dormant. Probably the ideal environment for growing these would be in a Mediterranean climate with only mild frost in winter. Coastal California and Oregon, southern Europe, Japan’s southern islands, or coastal Australia all seem perfectly suited to optimizing the growth and flowering potential of this species.

Typically winter frosts will kill the plant to the ground. Any temperature below freezing is strong enough to kill their water filled stems. This is not a problem for the plant since it has large subterranean tubers for storing energy for next year’s growth. If the soil remains unfrozen throughout winter, then this plant can be grown outside year round. In the eastern US it is said to be fully hardy to zone 8, and marginal in 7b. Along the coastal Pacific region of the US and Canada it can probably be maintained even up to Vancouver Island, though flowering it that far north could be tricky.

Tubers can be lifted in fall and kept in warm winter storage as with other dahlias. The problem comes in spring – the later you plant, the later it will begin growth, and consequently flower later as well. To get around this some growers have tried starting tubers early indoors – say around April and then planting them out once things warm up a bit. Presumably such plants will have a leg up on outdoor specimens and flower a bit earlier – perhaps before the first frosts hit. It’s a crap-shoot, but if you want to see flowers in a cooler climate you may have to try this technique. As a side note, attempting to grow these oversized plants in containers is not recommended since they are just too large, and will lose vigor unless planted in the open garden.

A somewhat odd “problem” with D. imperialis in areas that don’t experience frost is that old stems do not die down in winter, but rather continue growing and flowering up until Christmas time and even beyond. The problem is that older stems lose vigor over time (remember these are not woody plants), and have to be cut back after a couple seasons to keep the plants vital and flowering well – a “problem” I’m sure most northern gardeners would love to have!

They seem to be fully disease free once in growth, but I have noticed that recently planted tubers are given to rot. I’ve planted a couple that never took due to this, and I’ve heard other growers report similar difficulties. Once they get established however, this stops being a problem, in fact they tend to grow too well. I have seen a number of sap-sucking bugs infesting plants, leaf-miners going to town on their succulent leaves, but even this abuse doesn’t seem to faze well established specimens.

A commonly reported problem is that high winds break and deform their stems once plants get above 2 meters or so. This can be a real problem in coastal areas where winds are common, or in places that are subject to strong squalls or hurricanes. In my experience it is a good idea to support the plant starting in the fall. It doesn’t matter how you do this – wood bracing works fine, or wiring the stems to any firm structure will do the trick.

This past season I watched my neighbor’s large patch get beaten almost to the ground during an early fall typhoon. Luckily, my plant was protected by my house, so it took just a few support wires to keep it from flopping over. This tendency to flop over becomes more critical when the flower heads develop since they can get heavy, particularly during rain evens. The bottom line is you need to support your plants in the fall months for best results.

This is not a finicky species where it grows well. It requires full sun to grow and flower well, and a moderately rich soil that remains moist, yet well drained. Don’t worry too much about keeping the roots dry though since plants in southern Japan are subjected to 6 weeks of nearly unending rain during the summer monsoon, and don’t suffer in the least. You can fertilize them if you like, but as long as your soil is even modestly rich you’ll probably not want to encourage them to grown any bigger or faster – these are truly giant perennials! Pruning of the lead growth is NOT a good idea if you want flowers however, so just let them do their thing.

They can be grown from seed, or better yet, from stem cuttings after flowering is finished. Simply cut the stems into segments that have at least two nodes. You can completely bury these into the soil or plant them vertically with the lower node underground. Keep them evenly moist and above freezing. Eventually, they should root and throw new leaves – a fast way of increasing your tree dahlia collection.

Though not the perfect plant for every garden, the tree dahlia is a show stopper when in flower. If you live in a mild climate with warm summers and relatively long, warm autumns, then I’d give this one a try.