Tree Dahlia D. excelsa growing guide

Apollo Farm: Dahlia excelsa 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 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 soft cream brown with lime green leaves. Stems turn pale grey to brown after average six months and become more ligneous (woody), and heavy towards spring. Flowering from May to September each year with at least two flushes.

Expect 2m growth on first year, then cut down to the ground for vigour and even more flowers in the following seasons. Expect 4m growth second year. 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 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 a balanced fertiliser 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 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 it is best to divide the tuber clump. 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. A crowbar is useful to divide old tuber clumps. For very old tuber clumps a digger may be required.