Dahlias are grateful plants: the better you care for them, the better you will be rewarded with healthy growth and abundant flowers.  The tips we give below for taking care of your dahlias are general.  Different soil, temperature, light and feeding all influence the successful development of your dahlia tubers.  Simply by experimenting, everyone can find out what the ideal care is for dahlias under their circumstances.

Soil

Dahlias will grow in all types of soil.  It is important that the soil contains enough nutrients, drains well and is not too dry.  Soil acidity and alkalinity are measured on the geometric (logarithm not linear) pH scale, and can be done through a soil test. The log scale runs from 0 (pure acid) to 14 (pure lye). The neutral point is 7, which is neither acid or alkaline.  Very few plants will survive in soil more acid than pH 4 or more alkaline than pH 8.  Dahlias, like most plants, prefer a soil that has a pH of 6.5.  Decreasing soil pH levels can be achieved by adding chemicals such as sulfate of ammonia or sulfate of iron.  Increasing soil pH levels can be achieved by adding limestone (calcium carbonate).

Location

Always plant dahlia bulbs in a sunny spot.  Dahlias like all-day sun, although a few hours of shade a day is acceptable.  If there is too much shade then the dahlia plants often grow to tall and weak.  Lack of natural light will also cause a much less impressive flowering display.

Water

Dahlias require regular watering.  The soil should always be moist and not dried out.  Especially taller dahlia varieties can evaporate a lot of water through their abundant foliage on warm days.  Too little water causes slow growth, fewer flowers and yellowing of the leaves.

Feed

Dahlias require a good fertilizer.  Before planting the tubers, add for example some organic granular fertilizer (this is also environmentally friendly).  During the growth period you can regularly add some balanced fertilizer.  As with everything, don’t over do it, as this could cause more harm than benefit. Adding some plant feed every few weeks should be more than sufficient.

Temperature

Dahlias usually are happiest at temperatures between 15 and 25 Celsius.  As Dahlias are originally from Mexico, they are not frost hardy.  Therefore, do not plant the tubers until the threat of (night) frost has disappeared.  In northern Europe this generally is from May onwards.

Planting material

Dahlias can be sourced as tubers from early spring to early summer, or as seed.  By buying dahlia tubers you have the advantage of knowing which variety of dahlia you can expect to end up with when it is time to enjoy their flowers in summer.  Dahlias from tubers generally produce much bigger plants with much bigger and more impressive flowers than dahlias bought from seed.  Dahlia tubers have also been (health) checked and selected by size by the grower.  The largest size dahlia tuber is size I (size II and III being much smaller, and producing smaller plants and flowers). When you buy dahlia seeds, you will normally be buying mixtures of usually smaller plants with smaller, single flowers.  The variety in colour, flower type, plant height, etc can be significant, which makes it difficult to plan a border.   There are generally two types of mixed seeds “Mignon” and “Unwin”.  The package usually shows a mixture of flowers and would have to mention: “produced from seed”.  For those gardeners who want to have large flowers the first season and know what they are planting, buying tubers is usually the way to go. 

Planting period

As mentioned above, dahlia bulbs are not hardy.  If you want early flowers and can’t to start planting your bulbs as soon as possible, you can start planting the tubers 2 to 3 weeks before the risk of night frost has completely disappeared.  As the tubers are planted at a depth of around 5-10 cm, they won’t be out of the ground until the risk of night frost has gone.  Obviously, if you want to be certain to not kill off the young sprouts by any lingering frost, just be a little more patient.  Not every gardener has one, but if you have a small greenhouse, you could plant the tubers earlier in pots (from March), and then move the young plants to their intended spot in the garden when the weather is warmer.  This way you will be able to enjoy the flowers earlier.  Larger variety dahlias should be planted 40-60 cm apart and the smaller varieties 25-30cm.  Do make sure that the dahlias are watered frequently, especially just after planting (or moving).

On-going care of dahlias

Taller variety dahlias often need support.  The weight of the plant and flowers are often so large that they, especially with a lot of wind, they may fall and break. Staking the plants by inserting canes and tying the plants as they grow is easy and can usually be done very discreetly, given that dahlias produce ample foliage, which will hide the support canes.  The smaller pompon dahlias often do not need support, as they are lower and more compact.

To encourage branching, pinch out growing tips once the dahlias reach a height of around 40 cm.  If fewer but larger blooms are desired then restrict the number of flowering stems to three to five, for more but smaller blooms, allow seven to 10 flowering stems.  In order to prolong the flowering period and encourage strong stems, remove the two pairs of flower buds developing in the leaf axils below the terminal bud, and deadhead as flowers fade.

Dahlia as cut flower

Dahlias are perfect for use as cut flowers and on average should last about a week in the vase.  They come in many different bright colours, shapes and sizes, and have strong straight stems.  Remember that the best time to cut dahlias is early in the morning or in the evening, not in the middle of day.  As soon as you cut the stems, put in them in water to keep the stems strong.  You can always add some cut flower feed to prolong the life of the flowers. 

Unwelcome visitors

As with many plants and flowers, dahlias sometimes attract insects such as lice, slugs and snails, caterpillars, thrips, earwigs and red spider.  Check the foliage, stem, and flowers regularly and treat with the appropriate means.  There are several good environmentally friendly pesticides available at your local garden centre.

Should you wish to receive more information about dahlias then please do not hesitate to email Edward Pennings at info@bulbsandbeyond.com.