Dahlia Growing Guide

Ground Preparation;

This makes the difference between a good dahlia and a better dahlia.

Organic material is important for dahlias as they are a tuberous plant. They need large quantities of water and plenty of nitrogen. The best solution to this is to incorporate plenty of well rotted farmyard manure or rotted straw. Dahlia tops and straw from the previous season should be burnt on the growing ground, this has 2 advantages. Firstly any bugs and disease on this material will be destroyed by burning. Secondly the ash from the fire is a good source of Potash, the plants need this for flower production.

This material should be dig in early in the year once your growing area is clear of tubers. This gives time for the winter frost to do it’s business and break all the material down.

One month before planting the ground should be worked again to aerate the soil and break up any large clods. This is a good time to test the nutrient levels in your soil as it will give you time to rectify any deficiencies in your ground.

Lime, in acidic soils lime may need to be added. Ground lime may be added during the winter or hydrated lime may be added prior to planting. Dahlias need a Ph of between 6.5 and 7.

Finally once any deficiencies have been rectified a final cultivation just prior to marking the beds will clear any weeds and make planting a lot easier.

Planting;

Dahlias can be planted from either a tuber or from a young plant, it is very much upto your own personal preference which method you use. On our nursery we prefer to plant young plants, this method is also the method used by the majority of exhibitors in the UK. The reasons for this are that a young plant has more vigor and also stock is easier to select allowing for virus elimination and selection of plant characteristics.

Planting times vary depending upon which part of the country you are growing in. South of Birmingham planting can take place in mid May frost permitting. Further north up to about Newcastle planting commences in the first week of June. North of Birmingham planting often starts in mid June. These dates are by no means set in stone as Frost protection may well be needed to protect young plants.

From our experiences this year we now stagger the planting of our exhibition varieties. An early planting in June allows us to have good blooms for the shows in August and early September. A later planting at the end of June gives us blooms For mid September onward. It appears that planting a month later outside makes flowering about a fortnight later.

We plant in 3 foot(1M) wide rows with 2 feet between pairs of plants with one plant staggered in between. Support is provided by wire plant support netting in two layers approximately 18 inches apart. Stakes are placed every 10 foot. From our experience it is often a false economy to use for example stock fence or pig netting instead of plant support net. The cost of a roll may be greater but you often get far more on a roll of plant support net. In support of plant netting, it is easier to handle, and does the job it was intended for much better. Don’t skimp on plant supports it means the difference between plants that stand up and plants that fall flat in the first wind.

Once the beds are marked out a top dressing of a general purpose fertilizer such as Growmore or vitax q4 or for those organic gardeners bone meal, is all that is required. Gently work this into the top inch of the ground.

Plants should be placed in position prior to planting and gapped as needed.

Plant the plants to the top of the soil in their container and pinch out the growing tip to encourage bushiness.

Once the plants are planted into the ground it is advisable to lay seephose down the rows. It is far easier to do it now than when plants are taller and avoids you having to crawl between rows on all fours whilst avoiding to damage your plants.

Water the plants in well feeding is unnecessary at this stage.

Propagation;

Propagation is one of those areas that is very much dependent upon budget and available facilities.

To start with what sort of parent stock are you using;

To summarize parent stock generally may fall into one of 3 categories:

Pot Tubers

Field Tubers

Green plants

Pot tubers can be propagated in one of 2 ways, either they can be propagated from cuttings grown from the ctown of the tuber or they can be propagated by means of splitting the tuber.

Field tubers are propagated in exactly the same methods as pot tubers.

Green plants cannot be propagated by splitting. They can only be propagated by cuttings.

Division of tubers;

Tubers are divided by splitting the crown of the tuber so that the pieces that are split off have a section of crown with either an eye bud or shoot.

Propagation by cuttings.

Green cuttings are taken by cutting a growing shoot just below a leaf node with at least one pair of open leaves. This is then dipped in rooting hormone and placed in the rooting medium.

Starting plants into growth for the purpose of taking cuttings or planting out.

Tubers coming out of storage are in their dormant state. In order to break this dormancy they need 3 things, Heat, Light and Moisture.

Tubers should be placed on a heated bench and surrounded with compost in a frost free greenhouse. Water the compost and tubers and keep moist but not wet. The temperature of the bench needs to be around 15 Celsius growth will be evident in about 14 days. Mold and rot on tubers can be alleviated with sulphur powder.

This method of starting growth is also suitable for encouraging growth of split tubers. Tubers may often be desiccated when starting into growth moisture and heat should encourage growth provided desiccation has not gone too far. Pre packed tubers should be started into growth as soon as possible upon receipt leave them too long and they will deteriorate.

Rooting Cuttings;

Cuttings may be rooted in a number of ways but generally they have the same growth requirements as tubers namely, heat, light and moisture. Cuttings need far more moisture than tubers and a high level of humidity. For those without access to a mist propagation bench, a polyethylene tent or large bag to contain the moisture is adequate although rooting may well be slower. Spraying newly struck cuttings with a general fungicide will reduce rot and fungus growth on cuttings and increase reliability of rooting.

Rooting cuttings out of season can be awkward as dahlias are somewhat photosensitive, as day length shortens dahlias will produce buds. This means late cuttings are more prone to develop flowers than create strong basal growth. To alleviate this supplementary lighting used to increase day length to up to 16 hours is needed. The method we use is to suspend waterproof fluorescent lights 30 inches above the propagator bench.

Plant Care;

Feeding of dahlias is an area where everyone has their own opinion. From our experience a general purpose feed is needed during the growing season on occasion. Later as plants start to bud and flower a high Potash feed such as tomato food may be needed. Although generally plants need Nitrogen, Phosphate and Potash feeds contain trace elements in varying amounts. Certain plant feeds will work better for different plots as they may be correcting a deficiency in the soil this is probably the primary reason why there are so many differing views on feed requirements. Suffice to say that proper ground preparation is the key to success.

Apart from food the other major requirement of dahlias is Water. All living things need it and dahlias are no exception, dahlias are water hungry they need it to produce tubers and stems, if you cut through a stem you will see a large amount of water contained within it. At a guess I would say dahlia plants are composed of at least 95% Water. Don’t let plants dry out is the rule, dry plants don’t grow.

Once plants are growing away after being stopped shortly after planting place the first support net over them and stake it to reduce movement. Some varieties may need to be stopped again but it is dependent upon the individual variety. Once buds begin to form it is time to disbud, this involves removing the side buds from the top of the growing stem whilst carefully avoiding damaging the lead bud.

If you are growing blooms to exhibit it may be advisable to tie blooms to canes to prevent damage, use a long cane pushed into the ground that stops just below the bud and tie with pipe cleaners, twistits etc. Supporting of blooms is generally only needed for blooms greater than 6 inches diameter e.g. mediums and larger. If your site is windy it may be advisable to tie up smaller blooms. For garden blooms it is not really necessary to tie blooms as it can be a bit unsightly.

Pests and Diseases;

Dahlias are susceptible to a number of pests and diseases these fall into three groups,

  • Insect attack
  • Fungal attack
  • Viral problems

Insect attack

The more common problems seen include Whitefly, Greenfly and Blackfly. These can be controlled by spraying at regular intervals during the growing season. Effective controls are Sybol and Tumblebug used as per the bottle recommendations. Caterpillars can cause large scale leaf damage and are easily controlled with Sybol. A less common pest is Red Spider Mite, this upon inspection of the leaves has a number of characteristics. Leaves damaged by spider mite appear yellow and on close inspection of the underside appear covered in fine filaments of web, a magnifying glass is helpful here as mites are very small. Leaf hoppers leave characteristic small holes in the underside of the leaf whilst leaving the upper surface intact. Damage is often to the outer edge of the leaf and may appear as mottling on the leaf surface.

To summarize;

Deltamethrin or gazelle  for Aphids

Deltamethrin or gazelle for Caterpillars

Dynamec or floramite for Red spider mite

Deltamethrin or gazelle for Leafhoppers

The coice of sprays for insect attack is constantly changing. always read the label and use pesticides safely.

Fungal attack

The main fungal diseases of dahlias are mildew, this is caused by excessive humidity around the plants, this is the main drawback of covering plants as air flow is reduced. Powdery mildew appears as a silver sheen on the surface of the leaf, Downy mildew is more common under the leaf and appears as a clump of white pustules that will eventually eat through the leaf onto the upper surface. Suitable methods of control are Propicanazole and Aliette. Botrytis can affect dahlias during the propagation of new plants from cuttings, it is advisable to drench cuttings in fungicide prior to putting into the propagator. Following on after the wet season experienced in 2000 there appeared to be a large amount of Dahlia Smut about. This appears initially as a light green patch under the leaf that spreads to form a spot in the region of one quarter inch diameter, The affected area then goes brown and dies leaving holes of a similar nature to hail damage. The fungus that causes the damage over winters in the soil but does not carry on tubers. Infection is rife in damp humid conditions where water spray splashes soil onto foliage. Control is either with Bordeaux solution applied in June.  Sclerotinia can be beaten either through ground sterilisation or by the aplication of a pathogenic bacteria.  i have used both methods of control and found both to be highly effective. the method of choice depends upon the size of the area to be treated.

To summarize control of fungal problems;

Bordeaux mixture or copper based fungicide for control of Dahlia Smut

Propicanazole or Aliette for control of Mildew

Basamid sterilization good for sclerotinia, weed control, smut control and general cleaning of ground.

Contans WP, for control of sclerotinia gives good long term control over a number of seasons.

 

Viral Problems

These will tend to show up as a stunting of plants and leaves will appear contorted. Leaves often display yellowing and necrosis. There are a vast number of plant viruses that may affect dahlias, The ones that may be more probably seen are Tomato mosaic virus and Tomato spotted wilt virus, also it is possible that Potato mosaic virus could infect plants. Transmission of viruses is usually by one of two vectors; Thrips are carriers and also poor hygiene during taking cuttings e.g.. Using the same knife to take cuttings from infected stock and uninfected stock.

It is difficult to identify viruses, the most accurate way is to send a sample to a laboratory for analysis.

The only effective control method for viruses is incineration of affected plants. Certain dahlia varieties have been proven to be virus clear for decades and exhibit a natural resistance to viruses. breeding for natural resistance to viruses is the way forward for preventing future virus problems in dahlias.

known viruses detected on dahlia material

Tomato Spotted Wilt Virus, Cucumber Mosaic Virus, Tobacco Mosaic Virus, Dahlia Mosaic Virus. to name but a few

Protection;

This subject is concerned with growing dahlias under cover. Usually most exponents of this method use either a greenhouse, a poly tunnel or a removable cover. Each method has its advantages and disadvantages. The thing that all have in common is that bloom quality is greatly enhanced when growing under cover and plants get rather tall.

Growing in a greenhouse has advantages and disadvantages, for a start plants will often flower up to a month earlier and reach heights of 7 feet, use of stepladders is required to inspect blooms in this instance. Stem strength often suffers with wobbly blooms a problem. Temperatures in a greenhouse can be excessive this is the cause of thin stems where temperatures are on average 5 degrees above normal.

Growing in a polytunnel also brings the same benefits and problems as a greenhouse. With the added problem of condensation marking blooms. Also ventilation is an issue with higher incidence of mildew occurring under polyetheylene

Removable covers address the problems associated with ventilation and temperature but at the expense of bloom quality. They are also more likely to be damaged by wind especially if they are in areas that are exposed to wind.

The most important thing with growing under cover is to maximise the ventilation.

Lifting and Storing Tubers;

There are as many methods of storing tubers as there are growers growing dahlias. Every grower has their own pet method and the best advice I can give is if it works don’t fix it. Generally though the most common method is lifting of the tubers once the stems are dry and brittle, then remove any loose soil and store in boxes of peat in a dark frost free place. This method will suffice for most but lifting of tubers is probably more dependent upon the type of ground you are growing in. For heavy loams and clays it is wise to lift early, the ground will hold a considerable amount of moisture and tubers are susceptible to rotting due to excessive water around the crown.

For those with light sandy soils that are free draining lifting of tubers can be carried out much later provided the ground does not become waterlogged. On our nursery we lift tubers during the November but this is only really applicable to our particular ground conditions. 1 mile away the ground is heavy clay so tubers often have to be lifted up to 4 weeks earlier. we can also leave tubers in the ground under the tunnels up until January.