Hybrid & Shrub/Tree Dahlia Growing Guide

This growing guide applies to the following shrub form species and tree dahlia hybrids:

Dahlia tenuicaulis      

Dahlia apiculata                                       

Dahlia apiculata X tenuicaulis (Dr Melanie Gatt) 

Hybrid: Galaxy Prince (J Hall 2019) 

Hybrid: Timothy Hammet (Dr K Hammett) 

Hybrid: Blythe Spirit (Dr Hammett) 

Hybrid: Conundrum (Dr Hammett) 

Hybrid: Snow Prince (J Hall 2021)

Introduction

Tree dahlias are in vegetative growth mode early spring to late autumn. (September to May in NZ). Tree dahlias are equatorial, occurring in Mexico, Costa Rica and Guatemala. There are now cultivated tree dahlia hybrids of different form and colour as a result of plant breeding. 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.  Neither the cultivated D. imperialis nor the double white develop solid stems.  Their stems are cane-like and hollow. While they may persist for a second year, horticulturally they are better cut to ground level each year.”

Tree Dahlia ‘Galaxy Prince’

Semi woody perennial with very fine dissected leaves and long thin stems. Light and airy habit that looks perfect in a cottage garden. Flowers from May to August with rich mauve single flowers with a cerise halo. Grows 2.5m tall and 3m wide. Forms thin tubers 1.5m long. Requires excellent drainage and regular watering and mulching over summer, especially during the first year. Prefers full sun but can tolerate partial shade.  Very vigorous grower with many flowers. A great food source for bees in winter.

Plant in soil with excellent drainage or on a 30cm mound. Mulch 8-10cm, and water deeply every two weeks in summer if there is no rain. 90% of the matter of a tree dahlia is water. So, this hybrid benefits from regular watering and plenty of organic matter and humus in the soil. However, excessive nitrogen during the first six months of growth can cause root burn and produce thin weak stems. Remove leaves and the tops of stems. This will promote root growth and help establish the plant. Fresh new leaves will appear in about two weeks.

Fertilize with a seaweed solution or a balanced fertiliser once a month from March to June. Sulphur of potash 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 is helpful. This is an opportunity to inspect general health of the tubers. Check for rot in central tubers and constricted or twisted stem growth.

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.