Dahlia Growing Guide

Dahlia Growing Guide
Written by Joshua Hall, Dahlia Farmer. Edited by Dr Keith Hammett QSM

August

If you are growing dahlias for the cut flower market and need the flowers for Christmas sales, you should start growing your tubers this month in pots in a heated greenhouse. If you are growing dahlias from seed now is the month to begin germination in a greenhouse. Don't be in too much of a rush to plant them outside.

September

If you have a hot bed or heated greenhouse, you can start warming up the tuber clumps to produce some early shoots to take cuttings from. A 50/50 mixture of 1-4mm pumice and bark fines is ideal. The tubers don’t have to be placed in pots. Some growers use trays with a 10cm base of pumice, bark fines and sand and nest their tubers in this. A potting mix devoid of fertiliser will suffice. With some material around the tails and some support, the tubers will start to grow roots and shoots. Be careful not to damage the eyes and fresh buds by touching them as this could prevent a plant forming. Monitor regularly for hungry slugs and snails. You can take cuttings from fresh shoots as they grow.

Do not water tubers at this stage. They can rot and fail to produce shoots. After they have 20cm of stem and leaf growth they may become droopy. This is a sign that a light application of water is appropriate. If you have humid weather the tubers may not need watering at all. Err on the side of tubers and young plants being too dry.

October

This is the month to plant your dahlias in the ground in the North Island. A good rule of thumb is when the soil is warm enough to plant tomatoes, it’s the right time to plant dahlias. Remember dahlias are from Mexico.

Avoid nitrogen based fertilisers at this time. Fertilisers with a high nitrogen concentration can burn fresh roots and lead to long weak stems. Mixing in a mineral rich fertiliser and Epsom salts into the soil around your dahlias will promote strong stems, intensely coloured blooms and increase the overall health of your plant.

Depending on your reason for growing dahlias you will need to consider ideal spacing, staking and if you are going to use floral netting. Put stakes in the ground when you plant your tubers so you don’t damage the roots or tubers later on. You can attach cultivar name labels to the stakes. Dahlias can be planted as close as 40cm apart to create a hedge effect with their interweaving stems supporting each other.

Remember planting your dahlias on mounds will increase drainage and make digging them out later much easier. An initial 20cm mound is ideal. It will soon be 15cm after some rain. My mounds are 40cm wide to allow room for tubers to expand. Rows are 100cm apart with plants every 50-60cm. Numbered and labelled.

FAQ: Can I plant my "chicken leg" tubers directly into the ground?

Yes, planting direct is generally done at Labour weekend when the soil is warm enough, frosts and heavy rain have finished. When planting chicken leg tubers into the soil you want to position the tuber horizontally 10cm below the surface with the eye facing upwards but if you can't locate the eye, the shoot will find its way toward the light.

Dahlias can tolerate some wind. Light winds are helpful to reduce powdery mildew, fungal and bacterial infections. Some dahlia cultivars do benefit from support. Over the seasons you’ll learn how each cultivar performs in your garden. Some people grow dahlias simply to add colour and attract bees to their vegetable garden and may enjoy an informal planting arrangement. It’s a matter of aesthetics and purpose as to the ‘ideal’ set up.

Water the growing plants as required, but don't overdo it. In the early stages of growth if the soil becomes water-logged it can cause the hair-like feeder roots to rot. Water your plants at the base (like tomatoes) to avoid risk of fungal infection. Dahlias don’t like sustained high humidity. Once the plants have become bushy and over 1m in height it is a good idea to trim back the leaves in the first 30cm. This will increase air flow around the stems and the tuber crown.

November to March

Water your dahlias once every two weeks or if it’s very dry once a week during this time. In New Zealand dahlias generally don’t need fertilising until they are one foot high. However, if you are growing flowers for the Dahlia Society competitions in January and February then feed your dahlias earlier.

Pick blooms after dark or before 8am so that they are turgid and last longer in a vase. Take a water filled bucket with you into the garden. Dahlia stems are hollow. When placed straight into water, an air bubble will form preventing the stem from taking up water. Pierce the stem above your cut to release the air bubble. Change the water in your vases daily to increase the bloom lifespan.

Deadhead for continual blooms. Remove side buds to increase flower size and colour intensity and elongate stems.

Any dahlias with stunted growth should be removed as they are likely to be suffering from a virus. Some cultivars are more virus tolerant than others. Sterilize and clean your tools often to reduce spreading any pathogens.

April to May

Dahlias will begin to slow down now. However, some late or all season blooming dahlias will flower until the end of May. You can collect seeds from dried seed heads now. If seed heads are ripe but have not dried out yet, they can be dried in front of the fire or on a hot windowsill.

Powdery mildew can be an issue at this time of year. Spray any plants as soon as you observe it, to reduce the spread to other plants. If you haven’t already remove the leaves from the first 30cm of the plants to aid air flow. Monitor for pests, gall and fungal symptoms. Larger plants can be trimmed like a hedge. They’ll respond with fresh growth and flowers.

In May most cultivars can be cut back to 20cm above the soil. Do this on dry days when there is no rain forecast. Rain can get into the hollow stems and rot the tubers from the inside, especially Giants and Sunflower types. Cutting back makes the dahlia to put its energy into tubers not seed production. Leave them for two to three weeks so the tubers can cure (harden properly for storage over winter).

June to July

Dig up your dahlias now. Make your initial cut with a spade about a foot away from the main stem and continue right around the plant, this severs all the feeder roots and makes it easier to then dig up the clump. A fork may seem helpful but will likely pierce tubers you can see and create tension between tubers and roots resulting in broken necks and damaged tubers. A sharp strong spade is better as it will support tubers when you lift them. 

When cleaning tubers, it is important to reveal the tuber necks and crown connections so that you can clearly see the necks and the eyes. After cleaning leave the tuber clumps to dry out in a warm sunny area for 24 hours.

One of the most common methods of storage is a box, either cardboard or polystyrene, of damp sawdust, with tubers as "chicken legs" or full or part clumps. Your box should have drainage holes and be able to support your media. Fruit crates are ideal as they drain well and stack easily.

The media can be untreated sawdust, pumice or a fertiliser free potting mix. The media is there to moderate temperature and humidity swings and keep the tubers from drying out. Ideal Temperature for storage: 12-16˚C - Ideal Humidity: 40-60%. Humidity is important. Dahlias should not dry out over winter.

When you divide tubers you can create what are called "Chicken legs". These are single tubers which have been severed from the main clump with a small piece of the old stem still attached. It is important to make sure that each tuber cut off in this manner still retains material from the main stem, and that it has some viable buds (or eyes) present.

If you have free draining soil and have planted your dahlias on mounds it may be possible to leave them in over winter. Dahlias hate wet feet so don’t overwinter your dahlias in heavy clay soil. Be sure to cut them to the ground leaving at least one node above the crown. Cover the tubers with a bark mulch to protect them from frost and heavy rain.

Enjoy planning your Dahlia Garden!

©Joshua Hall. July 2021