National Dahlia Society of New Zealand

Blue Bayou Precocious Annamari Oreti Rebel Claire de Lune

What is the best way of obtaining tubers and when do you plant them?

Tubers can be obtained from garden shops, our tuber sales or commercial nurseries e.g. Belle Fleur gardens. Tubers from the garden shops are often available in late July which is too early to plant outside. Care needs to be taken in selecting these tubers which are brought in from overseas, and semi dried. They often have broken necks, .so make sure they have a good shoot before purchasing,. Sometimes they may not always be true to the label. They may also have to be started in a pot until the ground is warm enough.. Commercial nurseries are more expensive, but you know you'll get varieties that grow in our climate and they will replace any that do not grow or are not true to name. Of course our tubers are very realistically priced, with good shoots, but the particular variety you want may not be available!! Tubers shouldn't be planted out before Labour Weekend, as this is when all danger of frosts is past. Sadly its no guarantee that we won't get snow or hail!!! Ideally beds to plant dahlias should have been prepared 3-4 weeks earlier by digging over an incorporating a good general fertiliser e.g. Nitrophoska Blue. ( Warning this needs to be applied very sparingly a- small handful to a square metre is ample. Overfeeding leads to poor tubers in the winter ). It is a good idea to test the acidity of the soil before planting as the addition of manures and compost over the years turns the soil quite acid. Dahlias like a pH of 6.5-7. ( Soil testing kits or pH metres are available at Mitre 10 ) The addition of lime to the soil will sweeten it and improve the PH

What are pot tubers?

Commercial dahlia nurseries often supply pot tubers. These are treated like ordinary tubers and can be planted directly or cuttings or divisions taken from them. Pot tubers are produced from last years cuttings that are allowed to develop during the year in a small pot, ending up with a small clump of tubers ready to burst into growth the next year.

Does the size of the tuber matter?

No. Often smaller tubers grow better than large ones. Large tubers can be slow to produce roots and the shoot relies on food from the tuber instead of putting out its own root supply.

What do I do if my tubers show signs of decay?

Decay usually starts round the neck of the tuber. It is important to cut away as much of this as possible, using a sharp blade. If tubers show signs of rot at the ends this can be cut away and the ends treated with 50/50 mixture of Flowers of Sulphur and Hydrated Lime. Remember to sterilise your blades between clumps of tubers to prevent spread of disease. Blades may be dipped in bleach or flamed in Meths Good hygiene when dividing can help prevent tubers rotting in storage. Some growers dip the tubers in fungicide and/or insecticide before storage. ( Remember to wear protective gloves of doing this )

Is staking necessary?

Yes especially if you are showing. Even if you don't show a stake when planting is essential unless growing dwarf varieties. Alternatively several bamboo canes can be placed round the tubers tied with string as support. Some growers plant 3-4 tubers round each stake to increase the chances of getting enough flowers for show.

Should you water when planting?

When you plant be careful not to overwater. Tubers are liable to rot until they develop a good root system. However once buds start forming watering is very important. A good soaking once a week is better than small amounts applied frequently. Mulching is important to conserve moisture and keep the soil cool. Pea straw, pine needles are examples of mulches

What is stopping?

The idea of stopping is to produce a bushy plant with lots of blooms. Stopping is taking out the top bud allowing the leaf buds underneath to start growing. If you want to grow dahlias for show you will probably need more than one bush and ideally they should all grow in the same environment so they all look the same.

How and what do I feed?

Feeding is very individual. Some growers feed the soil during winter, some apply feed just before or while planting. Some use inorganic fertilisers and some use liquid fertilisers. Dahlias are heavy feeders. Look at the chemical breakdown on the label of the fertiliser. This is usually expressed as a ratio of Nitrogen, Phosphorus and Potassium (N, P, K. ) At the start of the season you'll want a fertiliser with a reasonable high concentration of Nitrogen to promote leafy growth, but once the buds form you want the plant to concentrate on producing flowers not leaves and you'll need to look for fertilisers with less Nitrogen . Phosphate promotes roots and blooms, while potash( potassium) promotes strong stems and tubers. Dahlias respond well to foliar feeding-apply product according to directions. The key to success is regular feeding.

What is disbudding?

Disbudding is the removal of buds to produce larger flowers. If you just want a garden display don't worry about this bit, if you want to show you will need to cut down on the number of flowers.

What can I use to control Pests and Diseases ?

Aphids are easily controlled by commercial sprays, but a regular spray programme is essential. Earwigs can be controlled by spraying about December and January with ICON or RIPCORD . These can be obtained from places like CRT or Reid Farmers. ICON comes as a single use sachet for about $15 while RIPCORD comes in a 500 ml bottle and is about $70 ( sounds a lot but lasts several seasons ). Orthene can also be used until flowering, and Carbaryl provides some protection too.

Sclerotinia is becoming more common. How do you recognise it?

Usually you will notice wilting leaves on the plant. If you look down the stem you will see a brownish watery area around the stem. If you cut off the stem and open it up and see blackish things that look like rat dropping you have Sclerotinia. Several commercial sprays like Greenguard or Watkins Fungus and Mildew Spray are effective against this, but again must be sprayed regularly. The club has just purchased several sprays specifically for Sclerotinia so if anyone wants any contact Robin 4773 669.

Why do you get Sclerotinia?

There was some thought that the fungal spores get in through any damaged area. Any wounds created near the ground ( e.g. branches broken off ) would provide an entry point.

Should we cover cuts etc?

One grower is trying a product called Trichoderma. This is available from garden centres and is reported to control Sclerotinia in daffodils. The ground is sprayed twice before planting, allowing the beneficial fungus to grow .

What spray do you use for mites?

Red spider mite can be controlled by sprays from the garden shop, but usually is only found in conditions of high temperatures and low humidity. Keep plants and soil moist.

What about mildew late in the season?

This seems to be related to the plants becoming damp during the cooler evenings. Good circulation of air helps, remove lower leaves from the plants to assist circulation. Water in the morning rather than at night.. Again a good spray programme will protect against this.

How can any member Identify or Name Existing Plants?

A good question. Bring them along to the next show and we could have a table for identification. Our more experienced growers are only too willing to try and identify them for you. There are several Dahlia books with good pictures that you could look through or perhaps take them to a commercial grower at the House and Garden Show or visit their gardens with a specimen.

In applying liquid manure is it best done in the evening or at dawn?

Most spraying or fertilising is better done in the morning allowing the leaves to dry during the day . This avoids the problem of the leaves becoming damp which may lead to the development of mildew.

What would cause "die back" in new plants-apart from lack of moisture.?

It's a little difficult to answer this as there could be several reasons. Too much water after planting can lead to problems, the shoots never develop, and when the tubers are exposed they are soft and decayed. Sometimes tubers can develop rot inside due to bacterial infection and the shoots never grow after this. Occasionally shoots will develop after planting but become hard and green and stop growing. Usually this is due to the tuber failing to produce roots, the shoot draws on the tuber for food but once the food supply is exhausted the shoot stops growing. In a more mature plant wilted leaves and stems can be caused by several fungal diseases. A watery brown ring around the stem which when opened shows signs of black "Rat droppings" means Sclerotinia is present. A watery ring covered with a white mould growth can mean Botrytis. In either case cut off the wilted parts below the rot and spray with fungicide or if very severe you might be better to pull the whole plant out and throw it away. Plants might start off looking healthy but become stunted with short internodes and deformed leaves. This usually means that the plant is "virused" and needs to be pulled out and disposed of immediately. A good motto to follow: "If in doubt throw it out".