Getting it to bloom

Why won't my Tree Dahlia bloom? (and how to make it flower)

Also called Tree Dahlia, Giant Dahlia, Bell Tree Dahlia, Imperial Dahlia (Dahlia imperialis).

More about tree dahlia

About Tree Dahlia

Dahlia imperialis · also called Tree Dahlia, Giant Dahlia · flowering

Dahlia imperialis is the towering species dahlia from Central America, capable of reaching 3-6 metres in a single season with bamboo-like hollow stems and clusters of lavender-pink, single ray flowers in late autumn. A dramatic architectural plant for large gardens. Dahlias are listed by the ASPCA as mildly toxic to dogs and cats.

Plant type: flowering

Watch for — Frost kill before flowering in cool climates: In the UK and northern US, late frosts often destroy flower buds before they open. Choose a sheltered warm microclimate and grow against a south-facing wall. In marginal areas, treat as an annual.

The reasons tree dahlia isn't blooming

Almost every non-blooming tree dahlia traces back to one of these, roughly in order of how common they are:

Too little sun — most of these need full sun (or very bright light) to flower well; shade gives leaves, not blooms.
Too much nitrogen feed, driving lush foliage at the expense of flowers (very common with general or lawn feeds).
The plant has not been deadheaded, so it stops flowering once it sets seed.
Irregular watering — drought or waterlogging at the budding stage makes buds abort.
It is still too young or was checked by a transplant and is rebuilding before flowering.

Feeding tree dahlia a high-nitrogen general feed and growing it in too little sun — you get a big leafy plant and almost no flowers.

The fix — how to get tree dahlia to flower

Maximise sun. Give tree dahlia the sunniest spot you have — for most bedding and fruiting plants, more direct light directly means more flowers.
Switch the feed. Move off high-nitrogen feeds and use a higher-potassium "bloom" or tomato-type feed as it comes into flower.
Deadhead regularly. Remove spent flowers often to keep it producing more rather than stopping to set seed.
Water consistently. Keep moisture even through budding and flowering — drought-then-flood swings make buds drop.

Light and feeding do most of the heavy lifting here. Dial in the spot with the light guide for tree dahlia and get the feeding right with the tree dahlia fertilising schedule — the wrong feed (too much nitrogen) is one of the most common silent reasons a healthy plant makes leaves instead of flowers.

Bloom season and what to expect

Tree Dahlia flowers across its growing season (mostly summer) and, kept fed and deadheaded, can bloom for many weeks or right up to frost.

Post-bloom care so it flowers again

Deadhead, keep feeding lightly, and many will rebloom; collect seed from the best plants at the end of the season if you want to grow them again.

For everything else this plant needs day to day, see the full tree dahlia care brief and its watering schedule — a stressed, badly watered plant rarely has the energy to flower at all.

Tree Dahlia blooming — frequently asked questions

Why won't my tree dahlia flower?

Tree Dahlia blooms on the season's growth given enough sun, warmth and the right feed — there is no cold or photoperiod trick, just good growing conditions and a bloom-leaning feed. The most common reason it is not happening: Too little sun — most of these need full sun (or very bright light) to flower well; shade gives leaves, not blooms.

How do I make tree dahlia bloom?

Give tree dahlia the sunniest spot you have — for most bedding and fruiting plants, more direct light directly means more flowers. Move off high-nitrogen feeds and use a higher-potassium "bloom" or tomato-type feed as it comes into flower.

When does tree dahlia normally bloom?