Tree Dahlia Growing Guide

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.

 

Species Tree Dahlias                           

Dahlia tenuicaulis                                 

Dahlia apiculata                                      

Dahlia imperalis (wild)                           

Dahlia imperalis (cultivated)     

            

New Zealand Tree Dahlia Hybrids

Melanie (Dr Melanie Gatt) Lilac Flower

Blythe Spirit (Dr Hammett) Red flower  

Conundrum (Dr Hammett) Yellow flower 

Galaxy Prince (Mr J Hall) Mauve/Cerise flower

D. excelsa and D. imperalis:

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.

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 month 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.

Cut stems to the ground in October when flowering has finished. A silky pruning saw is ideal. This is an opportunity to inspect the general health of the tuber clump. 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. They will quickly grow back. The crown region with the eyes is most important to preserve for future growth.

 

New Zealand Tree Dahlia/Shrub Hybrids

These hybrids have different parentage from the two species discussed above. They form large shrubs 2m wide and 2-2.5m tall. They do not require dividing and do not require cutting back to the ground each season. However they do benefit from hard pruning in late winter after frosts have finished. Remove stems which are rubbing on each other. Prune back to 60cm above the soil.

Galaxy Prince is a repeat flowerer. After flowering in autumn remove 30cm of the growth to promote flowering in Spring. 

Conundrum flowers from November to May so deadhead this dahlia shrub often for repeated blooms.

The same watering, soil, mulch and fertilizing mentioned above also applies to these hybrid tree dahlias.