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Your dahlia plant should be a showstopper, however, if you have dahlia leaves turning yellow, it’ll be a showstopper for all the wrong reasons. Thankfully, the discoloration is a warning from your plant telling you that it’s experiencing foliar distress.
Listen to it, find out what’s causing the leaves to yellow, make the corrections and your plant will start flourishing again.
Typical Causes of Dahlia Leaves Turning Yellow
The preferential soil acidity for dahlias is a pH of 6.5. Clay heavy soil affects drainage and when the soil doesn’t drain as fast as you need it to, the result is plant stress.
In the case of dahlias, the leaves turn yellow so the first check to do is a pH test / soil acidity test because if it’s over 6.5, the soil likely isn’t draining as well as it needs to.
Without correcting the acidity, the risk is waterlogging and that rots the tubers, eventually destroying the plant.
Chlorosis can strike any plant and it’s relatively easy to notice on dahlias. Leaves turning yellow but the veins remain green… that’s an iron deficiency causing a lack of chlorophyll production.
This problem can be traced back to the soil acidity because when the soil raises above 6.5 pH, iron becomes insoluble. If you’re fertilizing regularly and still not seeing improvements, it’s likely because the iron can’t be consumed by the plant because of the soil acidity.
Test the soil pH, and correct it to be slightly acidic.
The Most Problematic Pests that Cause Dahlia Leaves to Discolor
The potato leafhopper is prone to attacking dahlia plants in the early spring. These feed on the underside of leaves, cause leaves to yellow and curl and when a dahlia’s under attack from leafhoppers, they will not bloom.
Worse still is that as the name of the pest suggest, they will hop between leaves and between plants. So, don’t delay in treating your plants.
The more leafhoppers suck on the sap of the leaves, the more stress is caused on the plant. Insecticidal soap is the best solution to ridding dahlias of problem pests, but if you have none to hand, a good spray of water to rinse the majority of them off is better than leaving them all there to feast on the plant’s leaves.
There are a few types of scale insects that pose a high-risk to dahlia plants. None more so than the armored scale. The other types of scale insects are the common mealybug and the softback scale. The ‘armored’ part of the name relates to the rock-hard shell protecting the insect.
Scales are crawlers and don’t fly but they can still reach other plants within close proximity. Similar to potato leafhoppers, scales love to hide away under the leaves and will continually feed on them. The more they do, the more damage is done to the foliage.
An easy way to tell the type of scale that’s a problem to your plant is to look for honeydew excrement. This is a sticky substance that coats the leaves of plants and usually leads to black sooty mold.
The sooty mold interferes with photosynthesis and by that stage, the leaves on your dahlia will be turning yellow.
When scales are present and there’s no honeydew or sooty mold, it’s likely it’s the armored scale that’s invading your plant as that’s the only hardback scale insect that doesn’t leave a trail of honeydew in its wake.
The shells on armored scales protect them from a number of insecticides making these the most difficult to get rid of. When scales are present, the plant needs to be isolated, and treated with an insecticidal soap or fungicide.
Armored scales are the hardest to get rid of but it’s not impossible when you know the science. According to Nurserymag.com, there are three neonicotinoids that can be effective at ridding infestations of armored scales.
Look for insecticides containing any one of those ingredients. As armored scales are protected by a shell, they’re resistant to water and many insecticides so the key to getting rid of them is feeding them toxic chemicals that will kill them, preferably before they kill your dahlia.
Thrips are another hardy insect that’s a nuisance to control because when you have one, you have an infestation. These are the gangsters of the plant kingdom and that’s because they’re fast breeders.
Thrips are tiny to spot as their size can be as small as 1/25th of an inch. Adults are winged too so they can fly to other plants, infest them and bury even more eggs inside other plants.
Worse still is they can cling to the leaves so a blast of strong water from a hose or sprinkler isn’t always the best option. It can knock them off of the plant, but it can also damage the plant in the process by causing tears in the leaves, opening up the risk of bacterial infections.
Rather than treating your plant with random insecticides in the hopes that you can get rid of thrips, it’s best to take a targeted approach.
Insecticides that contain beauvaria bassiana are ideal for targeting thrips, whiteflies, and aphids, each of which can effect dahlias, but more so thrips because these are attracted to the light-colored plants, in particular, white.
The Need for A Proper Mulching!
Dahlias hate cold and wet soil. Organic mulch is the solution. It keeps the temperature and moisture level closer to constant. It’ll never be exact, but it will better prevent them from drying out or drowning.
Yellowing on the leaves of dahlias can happen as a result of overwatering and also from getting too much nitrogen.
A common problem with dahlias not blooming is the use of a nitrogen rich fertilizer. They do better with less nitrogen, but, if you’re using a wood-based mulch, a nitrogen deficiency (caused by the natural decomposition of wood) can cause yellowing.
If you have your dahlias planted in a garden bed that’s surrounded by a wood-based mulch, an increase in nitrogen could be just what’s needed.
As an example, gardeners focused on getting the best blooms from dahlias may opt for a 5-10-10 fertilizer to decrease the nitrogen, but if it’s already getting less from the organic mulch decomposition, it may be too little that it’s struggling so therefore needs that boost to return its green foliage.
Preventing the Overwatering of Dahlias
When there’s no obvious signs that point to why you have dahlia leaves turning yellow, the only thing left to consider is watering. Over watering any plant usually leads to leaf discoloration. The same happens with dahlias.
Dahlias do best with a watering that’s equal to 1-inch per week, but that is provided the soil is well-draining. If the soil is clay heavy it won’t drain as fast as you need it to so even just an inch of water a week could still be too much.
Both underwatering and overwatering dahlias will cause leaves to yellow and wilt. Edge on the side of caution and cut back on watering, increase organic mulch around the dahlia to retain more moisture and temperature and keep a close eye on the underside of the bottom leaves for any sign of pests that are likely to stress the plant.