Tree Dahlia Growing Guide: D. excelsa

 

Introduction

Tree dahlias are in vegetative growth mode early spring to late autumn. (September to May in NZ). Be careful of frosts in spring especially if growing from seed. Tree dahlias are tender and do not tolerate frosts unless they are well established with extensive tubers and deep mulch as protection. Tree dahlias are equatorial, occurring in Mexico, Costa Rica and Guatemala; penetrating and severe frosts and snow will kill them.  But are these dahlias really trees?

Dr Keith Hammett explains further: “The definition of a tree is not simply a matter of height.  The essence of a tree is that each year's growth is made as a layer over previous years' growth.  Hence the annual rings we see in logs.” Tree dahlias are in fact hollow and therefore not true trees.

Tree dahlia: Dahlia excelsa

This is the toughest of the tree dahlias species. D. excelsa 'Double White' has white anemone flowers from July to September and is a strong garden statement. Stems can be 80mm thick and are almost completely hollow. These stems/canes are robust and can tolerate 45kmph winds. Canes are lime green turning soft cream brown with lime green leaves. Stems become more ligneous (woody), and heavy towards spring. Flowers occur from July to September each year with usually two flushes.

Grows 4m tall on average each season. For maximum growth amend the soil with plenty of compost, gypsum and mulch. Tree dahlias like a good drenching of water every 2-3weeks in summer. Tubers grow to 2-4m, are elongated and thin.

Anemone flowers are plentiful and occur at the top of stems so a ladder is required to pick them. Best picked in stem bunches.

Plant in soil with excellent drainage on a 30cm mound. Mulch 8-10cm. Water deeply every two weeks in summer if there is no rain. 95% of the matter of D. excelsa is water. This species benefits from regular watering and plenty of organic matter and humus in the soil over summer. Excessive nitrogen during the first three months of growth can cause root burn and produce thin weak stems. Fertilize with a seaweed solution or blood and bone once a month from Nov to April. Sulphur of potash slow release fertiliser should be applied in February to strengthen stems and increase the number and solidity of blooms later in the season.

With all tree dahlias it is best to cut stems to the ground in October when flowering has finished. A chainsaw or silky saw is helpful. This is an opportunity to inspect the general health of the tuber formation. Check for rot in central tubers and constricted stem growth.  After three seasons divide the tuber clump to remove tuber rot and chocked tubers. Tubers can be 4.5m long but don’t be afraid to cut them with a sharp spade to manageable size. The crown region with the eyes is most important to preserve for future propagation.

Species Tree Dahlias                                       New Zealand Tree Dahlia Hybrids

Dahlia tenuicaulis                                             Dahlia apiculata X tenuicaulis (Dr Melanie Gatt)

Dahlia apiculata                                                Blythe Spirit (Dr Hammett) Red flower   

Dahlia imperalis (wild)                                   Conundrum (Dr Hammett) Yellow flower

Dahlia imperalis (cultivated)                        Galaxy Prince (Mr J Hall) Mauve flower