8 Reasons Your Dahlia’s Leaves Are Turning Yellow and How to Fix it!
If your dahlia's leaves have started turning yellow, you might be a bit concerned. This is usually a signal of many different potential problems. But, the good news is many of these issues are reversible. In this article, certified master gardener Liz Jaros examines the most common reasons this may happen, and how to fix it.
It’s halfway through summer and the dahlias have begun to dance in your yard. Bold, fringy blooms stand tall in a kaleidoscope of colors. Showy, oversized flowers sway gently on long stems in the warm breeze. Your senses are delighted each time you go down the garden path. It’s practically perfect. Except for those curling yellow leaves on your precious collarettes. When did that happen? And the pale, floppy stems on your pompons. What’s that all about?
Botanically speaking, chlorotic is the term used to describe a plant leaf that lacks chlorophyll. When green tissue turns yellow, it’s an indication that something has disrupted the process of photosynthesis, and it can be caused by a number of issues.
For seasoned dahlia growers, yellow leaves are not an uncommon observation. The genus is known for having a temper tantrum here and there. But if you’ve spent a long, frosty winter dreaming about their return to the yard, and a looooong, chilly spring giving them a headstart in your greenhouse or kitchen window, these tell-tale signs of plant distress can be particularly upsetting.
Fortunately, yellowing leaves might be the first indication that something is wrong with your dahlias, but they aren’t necessarily a death sentence. Let’s take a look at some of the likely reasons your dahlia leaves are turning yellow, determine if there’s still hope, and consider what might be done to give your ailing tubers a fighting chance.
Dahlias need even, consistent watering in order to thrive. They should never dry out, but they should never be drenched. Sounds like common sense, I know, but achieving proper irrigation can be a little tricky.
If your yellow dahlia leaves are droopy, soft to the touch, and have brown tips, you’re probably overwatering. Tuberous roots can get waterlogged quickly, and their vascular systems can be clogged by muddy conditions. Overwatering also opens the door to issues like root and stem rot.
If leaves are crunchy and dry, you may be underwatering your dahlias. Tuberous roots are also highly susceptible to drought and the first above-ground sign of distress will be dying yellow foliage that curls under at the margins.
A simple finger poke can give you a good idea of the moisture content below grade. Soil should not be noticeably wet any deeper than two inches down. If it is, you might want to dial back the watering a bit. If the top two inches are crumbly and dry, you should bump it up.
Dahlias require about 1 inch of water per week while they are establishing, and roughly 2 inches of water per week while they are blooming. Depending on your grow zone and precipitation rate, this means you should be watering somewhere between 1 and 3 times per week. Ideally, use a slow soaker hose or a gentle spray as close to the soil as possible, and water in the morning so leaves will have a chance to dry out before nightfall.
Poor Soil Texture
Dahlias grow best in well-drained soil that’s loose and crumbly. They are particularly unhappy in garden beds with a heavy clay content since their tuberous roots are easily clogged and highly susceptible to rot.
To determine if your yellowing dahlia leaves are the product of poor soil makeup, dig a 6-inch hole and grab a handful of dirt. Make a fist. If soil binds together in a ball and feels greasy, you’re probably dealing with clay soil.
Adding compost and organic matter (dead leaves, peat moss, sand, straw, shredded newspaper, wood chips) to beds that are clay-heavy can go a long way toward aerating the soil and creating a more hospitable environment for dahlia roots. But the process requires patience and can take years, and the ideal time to amend clay soil is well before you’re ready to plant.
If you believe your dahlia leaves are yellowing due to poor soil texture, you can try working some organic matter into the top six inches of soil (carefully!), and you can try poking some holes in the soil around your stems (very carefully!) to create a more porous environment.
If it’s early in the growing season, you can try transplanting your dahlias (extremely carefully!) to a bed with better drainage. But the best thing you can do for future dahlia success is to start working on next year’s beds right now.
If you’re still scratching your head over yellow dahlia leaves, you may want to test your soil’s pH levels. Acidity in the soil affects root growth and water uptake as well as nutrient content and decomposition.
Evaluated on a scale of 0-14, a soil pH of 7 is considered neutral. Soil with a value above 7 is considered alkaline, while soil with a value below 7 is considered acidic. Dahlias thrive in beds that are slightly acidic, with a pH balance of 6.5 being the sweet spot.
If you’ve determined your pH is too low, consider working some lime into the garden to tone down the acidity. Available in powder, pellet, or liquid form from your local garden center, lime should be raked into the top 6 inches of soil and applied according to the manufacturer’s directions.
If pH is too high, sphagnum peat moss or sulfur products can be used to increase acidity. Again, the ideal time to correct soil structure is before planting, but you can try working these materials around established dahlias. At the very least, your beds will be in better shape for the next growing season.
If your dahlia leaves have green veins with yellow leaf tissue, they are quite likely suffering from some kind of nutrient deficiency. All of the issues we’ve previously discussed (watering, soil texture, and pH balance) can affect your soil’s nutrient content and positively or negatively impact plant health.
Again, the proper way to confirm your suspicion and plot your correction is through a soil test. But there are a few signs that may indicate your dahlias are suffering from one deficiency over another.
Playing a key role in the photosynthetic process, magnesium is the most common nutrient deficiency in the dahlia garden. If you notice older leaves suffering before new growth, your soil may be lacking magnesium. Consider working a product with epsom salt into the soil, and continue amending each year with magnesium-rich organic matter.
If you notice newer leaves yellowing before older growth, an iron deficiency may be the culprit. Iron plays a major role in the metabolic process and contributes to a healthy, green leaf structure.
A product containing sulfate or chelate can be added to established dahlias to boost iron in the soil. This can be done through a soil supplement or foliar application. Reducing a soil’s pH will also help encourage a plant’s iron uptake, so you can try working a little peat moss into your beds.
Since high potassium levels can interfere with both magnesium and iron uptake, consider using a fertilizer that dials down that nutrient a bit if you suspect an issue with either of these nutrients.
Dahlias are susceptible to a number of insects, and yellow leaves can definitely be an indication that this is happening in your garden. Let’s take a look at some of the more common dahlia pests you might have to deal with.
Snails and Slugs
Yellow leaves with irregularly shaped holes or shredded margins typically indicate that slugs or snails have taken a fancy to your dahlias. If you suspect snail/slug damage, take a flashlight out to the garden at night and examine the undersides of your leaves. You’ll likely see them creeping around.
Dahlias are most at risk to these pests early in the season when foliage is tender and easily munched, so it’s important to get out in front of snails and slugs. You can try setting a beer trap (a cup of cheap brew buried in the garden will draw these chewers in and drown them) or deter them with coffee grounds, eggshells, chives, mint, or garlic. A spinosad-based product like Sluggo can also work, but the most effective way to rid your dahlia beds of snails is diligent hand picking.
Yellow, distorted leaves and a sticky black substance on your dahlias might be an indication that aphids have settled in. These small sap-sucking insects can do a lot of damage and harbor disease, so it’s important to address them promptly.
Pesticides are typically less effective than natural techniques for discouraging aphids and often cause more harm than good by killing predatory insects. You can try spraying a hard stream of water on the undersides of leaves, where aphids congregate, or coat leaves with horticultural oil.
Introducing predatory insects like ladybugs and lacewings to the garden can help keep the population down. Garlic, chives, nasturtium, and catnip can also be effective in repelling aphids.
These pests will leave little black puncture wounds on your dahlia’s leaves and create a stippled, yellow pattern. When thrips are suspected, try forceful hose spraying several times a day. Or coat leaves with neem oil.
Mottled yellow leaves that curl up and have a sticky web presence are likely infested with mites. Too tiny to see with the naked eye, mites typically work from the ground up, so removing the lowest leaves from your dahlias can help discourage an upward spread. Applying horticultural oils or soaps on the undersides can be effective, or try an abamectin-based insecticide. Plants that are overrun with mites should be destroyed to prevent spread.
Most common dahlia diseases are related to poor growing conditions. Starting with a healthy foundation of nutrient-rich, well-drained soil is the best way to encourage bountiful, beautiful blooms that last long into the fall. When yellow leaves are caused by disease, it’s typically fungal in nature and can be attributed to a breakdown somewhere in your cultural practices.
This soil-borne fungal disease will present with circular yellow spots that turn brown and dry out. Dahlias grown in high humidity are particularly vulnerable to dahlia smut.
Transmitted mainly through the soil, this one is not necessarily a death sentence for dahlias in pots, as the dirt can be disposed of after the growing season. If your dahlias are in beds, however, you’ll want to remove them promptly and dispose of the soil where they were grown.
When this nasty fungus takes down a dahlia, its leaves will turn yellow for a few days before the whole plant turns brown or black. Verticillium thrives when temperatures change quickly from cool to warm, so look for it in zones where the weather can be wacky. If you believe your dahlias are suffering from wilt, get them out quickly along with the soil in which they’re grown.
Stunted, twisted leaves with yellow streaks near the midribs are a telltale sign of viral infection. While there are several viruses that can wreak havoc in the dahlia garden, Mosaic Virus is one seen most often. This deadly dahlia foe is typically transmitted by aphids or dirty tools, and plants that are afflicted will need to be removed promptly.
To prevent mosaic virus (or any other suspected virus) from spreading to other plants or infesting next year’s crop, keep your tools sharp and sterilized, only propagate from healthy plants, and have a strong aphid management plan.
When the days grow shorter and a chill returns to the air, your dahlias will begin signaling that this year’s show is over. Leaves will begin to yellow and droop, and blooms will drop off without being replaced. If you’re planning to save and/or propagate your tubers for next season, wait until the first frost to dig them up and dry them off. They need this time to conserve some energy for dormancy.
Even for seasoned horticulturalists, determining the cause of yellow dahlia leaves can be a challenge. The biggest tool you have in the fight against plant distress is the power of observation.
With boisterous blooms catching your eye left and right, it can be easy to get distracted and forget to check under the hood. But if you devote a portion of your maintenance routine to examining leaves, if you employ sound cultural practices, and if you know what to watch for, you can get out in front of most issues and hopefully head off some heartache.