How to grow dahlias in hot weather?

Choose the right location

The location is crucial for the growth of dahlias. They will only be happy and floriferous in a warm, sunny position. Dahlias grow all over France. The sine qua non condition for having thriving dahlias is to plant them in the sun. However, in southern regions, avoid full sun and prefer to provide them with indirect or filtered sunlight, especially during the hottest hours of the afternoon. A shaded spot that receives morning sun and partial shade in the afternoon would be ideal.

Dahlias love sun and heat!

Pamper the soil

To bloom well, dahlias prefer soil rich in organic matter and well-drained. They enjoy light soils that remain cool and sufficiently moist with good mulch during the growing season. A few days before planting, add an organic fertiliser such as compost or well-decomposed manure to increase the organic matter content of the soil, which will also help retain moisture. In heavy clay soils that retain water, incorporate coarse sand or gravel to improve drainage, as conversely stagnant moisture would encourage tuber rot.

Pamper the soil at planting

Water wisely

During the growing season, the dahlia is thirsty for water, especially during the hot summer days! Throughout the beautiful season, pay particular attention to its water needs. Water generously during the growth of the buds. Deep watering, once or twice a week, is preferable to more frequent, lighter watering. Ensure that the water penetrates deeply into the soil to reach the tubercles. Always water at the base of the plant and not on the foliage to avoid certain diseases such as powdery mildew. Prefer watering late in the evening or early in the morning to avoid water evaporation. During a heatwave, increase the frequency of watering, don’t hesitate to water daily while avoiding flooding to prevent tuber rot, as dahlias dislike having their roots constantly wet.

For dahlias in pots, water as soon as the substrate becomes dry; it should remain moist. Consider using a potting mix (Grain d’eau) made from turf, which increases water retention capacity.

Mulch

An organic mulch is particularly useful in hot weather to help retain soil moisture. To maintain humidity between waterings, consider mulching around the base (hemp or flax pale, a layer of lawn clippings, or compost), to a thickness of 6 to 8 cm, leaving space around the stem to avoid the risk of rot.

Fertilise wisely

Dahlias are greedy feeders, but they are sensitive to over-fertilisation, especially with high-nitrogen fertilisers, which can do more harm than good in hot weather. Use a balanced slow-release fertiliser that releases nutrients over an extended period. Organic fertilisers are also an excellent option. To prevent foliage development at the expense of flowers, limit nitrogen inputs!

In pots, the substrate depletes more quickly than in open ground, so add a fertiliser for flowering plants to the watering water every 15 days from June to October to support flowering.

Shade them!

Create shade to protect your dahlias! Provide them with a plant cover using taller plants that will shield them from the sun’s rays and thus reduce evaporation. You can also place a shade cloth or a parasol above your plants during the hottest hours of the day.

Cut the faded flowers

This will prevent your dahlias from unnecessarily depleting their resources! Anticipate: make bouquets! Well-suited for vases, dahlias create wonderful summer flower arrangements that are very colourful. Some varieties, such as the Dahlia ‘Karma Lagoon’, have been specially developed for cut flowers.

Regularly cut the flowers to encourage new buds

Keep an eye on them!

Mulch, water, and feed your dahlias regularly, but above all, keep a close eye on them. In hot weather, symptoms can appear quickly. If you notice signs of stress such as browning or wilting leaves, act swiftly to rectify the issue.

For further reading

• Discover everything you need to know about the Dahlia in our comprehensive guide.