Although dahlias are resilient and can tolerate all but very difficult conditions, they are vigorous growers and naturally gross feeders. The highly-bred cultivars that produce huge numbers of amazing blooms are especially gluttonous.
Incorporation of organic materials and fertilisers into the soil before planting tubers is the greatest key to success. After planting into fertilised soil do not add food for about eight weeks although you can scatter some pelletised slow release fertiliser around the plants – it will break down progressively as the plants, also progressively, begin to demand extra nutrition.
Any fertiliser designed for flowering plants will produce good results. Once the plants are up to a good height and starting to form buds avoid anything that is high in Nitrogen – it will encourage leaf at the expense of flowers and also affects the keeping quality of tubers. Again, organic fertilisers in pelletised or liquid form that include seaweed and fish products are ideal for promoting root growth and healthy soil as well as flowers.
A liquid plant food that is high in Phosphorus and Potassium and low in Nitrogen used as a foliar feed sprayed on to the leaves allows the plant to take in nutrients quickly. This is particularly useful if the soil pH is high (alkaline), locking up nutrients in the soil. Regular low-concentration feeding (“weakly weekly”) gives the best results.
Towards the end of the growing season a feed of a high potassium fertiliser will help the tubers plump up and ripen, improving their prospect of keeping well during their dormancy.
The exact watering regime used will depend on soil type, rainfall, temperature and the size of the plant.
The advice to plant into moist soil and then avoid watering until the plant is 15cm high needs to be tempered by consideration of the conditions. In dry weather and in free-draining soil the plant might not get there! Check the soil occasionally and water lightly if necessary to keep it moist.
Watering infrequently (say twice a week) and thoroughly works well in moisture retentive soils. The tuber produces feeder roots that extend up to 60cm but do not go deep so daily or twice daily light watering is generally preferable in lighter soils.
Pulse watering - short bursts of watering at ground level (not on the leaves) at frequent intervals throughout the day has been shown to help protect the plants from dehydration and sunburn. A drip system is more water wise than overhead watering and a timer-based system that can be overridden if required to suit varying conditions is a great labour and worry saver.
Avoid watering buds and flowers as this can cause spotting, spoiling the blooms. However spraying water on and under the leaves helps control red spider mite and can be useful in severe weather.
A good mulch will protect the soil and conserve moisture. Pea straw and lucerne mulch replenish soil nitrogen as they break down and when they are incorporated into the soil at the end of the season.
Recent years have seen significant increases in the incidence of plant damage caused by extreme weather events.
Hot sunny spells can scorch buds and discolour blooms, especially large red and purple blooms, if they are in exposed positions. Umbrellas or other shade such as old sunhats mounted on stakes will protect the more sensitive buds and blooms.
Extra watering in hot weather has also been demonstrated to reduce damage. Frequent short bursts of drip watering during the day are effective. Despite the normal gardening advice to avoid watering green leaves when it is sunny and hot, some growers have found doing so actually reduces foliage damage.