Drive through any suburb or pass a roadside store, and you shouldn’t be surprised to see zinnias featured on border walls and in pots along driveways. It’s no wonder they’re one of the most popular annual flowers grown. While zinnias are generally easy to grow, they’re not immune to certain problems in the garden.
These are crowd-pleasing flowers. They are low-maintenance, come in various colors, shapes, and sizes, and are safe for gardeners, pets, and wildlife. Plus, they’re a great beginner flower if you’re new to gardening! This easy reputation can be frustrating if you experience problems growing zinnias in your garden.
To help you have the best experience growing zinnias, here’s a guide including cultural mistakes, diseases, and pests and how to combat them. Let’s dive in.
Cultural Mistakes
Overwatering
Water your plants in the cool morning hours to allow moisture to absorb or evaporate off during the daytime.
This problem results from fussing over your zinnias too much. Too much water can drown the root system, leaving it unable to properly absorb nutrients and photosynthesize.
Symptoms: Wilting, brown tips of leaves, lesions, yellow leaves, root rot, soggy soil
Solutions:
- Once your plants are well-established, they may only need to be watered if you are in a period of drought. Check on them every few days (or more in high heat) to determine if they need more water.
- Check the soil often using the finger test. Push your fingers about two inches deep into the soil around your zinnias. If it is dry two inches down, it’s time to water.
- Water your plants in the early morning before temperatures have risen too high so the water will have time to absorb. If you water at night and the soil or leaves are left damp, disease risk increases greatly.
Underwatering
Use a basic moisture meter and a rain gauge to monitor the moisture in your garden.
Just as overwatering causes zinnia problems, so does underwatering. Zinnias are fairly drought-tolerant, making them a great option for southern and warmer zone growers. However, they need water to grow like everything else, so don’t forget about them when planning your watering schedule.
Symptoms: Floppy or weak stems and flower heads, dry soil, stunted growth or death
Solutions:
- Young zinnia transplants need more water in the early growth stages. Without this, growth will be stunted, or they could perish. Water up to three times a week as needed.
- Install a simple moisture meter and a rain gauge to track how much rainwater is naturally provided to your garden. Then, you can plan how much you need to irrigate.
- Automate your drip irrigation by installing a timer and setting it on and off at certain times of the day for efficient absorption. You can change this schedule throughout the year depending on how much rain you get and as your zinnias grow.
Doubles Reverting to Singles
Prevent transplant shock or other plant stressors to avoid reverting to a single bloom.
Nothing is worse than selecting a beautiful, double-bloom zinnia only to have it revert to a single, which can happen if a flower is stressed. This is because the plant is spending energy on simply surviving, which takes away from the energy sent to the bloom.
Symptoms: The plant only produces a single bloom rather than the expected semi-double or double bloom.
Solutions:
- Prevent transplant shock. Be very cautious when transplanting in the spring/early summer if you grow in a cooler growing zone and during peak summer if you grow in a warmer one. Soil temperatures that are too low or hot, sunny days will cause high stress upon transplanting.
- Do not under or overwater.
- Space accordingly, especially if you have only ever grown single blooms. These will need a little more space. If you typically plant at six to nine inches, try giving double blooms 12 inches.
Planting Too Densely
Ensure proper spacing of your zinnias to promote sufficient airflow and minimize the chances of airborne diseases.
Although it can be tempting to squeeze more flowers into a small space, you’re doing yourself a disservice when you do. You’ll have healthier, happier, and longer-lasting plants if you give them enough space to thrive.
Symptoms: Small blooms, no blooms, early season disease.
Solutions:
- Ample airflow will help decrease the risk of airborne diseases, so space out your zinnias properly. This will depend slightly on the variety as larger blooms require more space, but on average, you should space your zinnias at 6-12 inches and three to four across in a 30-inch bed.
If you are planting them along a border, use similar spacing as above, and stagger the colors and heights if you desire an interesting, eye-catching garden feature.
- Plant zinnias in pots or fabric grow bags and move them around throughout the summer to change up the look of your garden.
Fungal Diseases
Powdery Mildew
If not addressed, powdery mildew can lead to wilting, stunted growth, and leaf drop.
A common zinnia problem, powdery mildew (PM) spreads very easily and affects various vegetables and flowers.
Scientific Name: Erysiphe cichoracearum
Causes: Periods of extended high humidity and standing water on leaves, which can include overwatering. Powdery mildew thrives in temperatures between 60-70° and can often be exacerbated by a lack of airflow around plants, as this prevents leaves from drying out after watering or morning dew. The damp leaves cause mildew spores to stick rather than blow past in the breeze.
Symptoms: Whitish gray powdery spots mostly across leaves, sometimes on other plant parts. Not to be confused with downy mildew, whose spots are yellow. Powdery mildew can also cause wilting, stunted growth, and leaf drop if left untreated.
Solutions:
- Select resistant cultivars.
- Although the fungus causing PM dies when the host plant dies, mycelium survives by laying dormant and overwintering, so it’s important to remove all plant debris from your garden. If you know the disease is present, move that plant debris to a burn pile rather than the compost.
- Take extra caution during warm daytime temperatures and cool nighttime temperatures when this fungus thrives.
- Spray copper or sulfur-based fungicides when disease is detected. Spray thoroughly and repeat every seven to ten days to prevent further infection.
- Plant zinnias in full sun so the plants can dry off easily from irrigation or morning dew.
Botrytis Blight/Gray Mold
Gray and black fuzzy mold spots on plants are an indicator of Botrytis.
Botrytis is sometimes referred to as gray mold because this fungus causes gray and black fuzzy mold spots on plants. It causes problems with zinnias most when running rampant in enclosed greenhouses.
Scientific Name: Botrytis cinerea
Causes: Most often by Botrytis cinerea, but other species of Botrytis may also cause problems.
Symptoms: Damping off, fruit rot, cankers, fruit blight, root rot. Lesions may begin as brown, water-soaked areas and, as they mature, turn gray to black.
Solutions:
- Don’t overhead water. Instead, water at the roots in the early morning and avoid doing so on cloudy days.
- Space zinnias out to provide good airflow. If you are growing in a greenhouse, provide air circulation and ventilation for overall health and to keep the relative humidity at a healthy level. The continuous air movement will prevent wet leaves.
- Soil test regularly. High nitrogen can promote fungus growth.
- Remove plant debris swiftly.
- Spray copper or sulfur-based fungicides when disease is detected. Spray thoroughly and repeat every seven to ten days to prevent further infection.
Alternaria Blight
At the first sign of early blight, combat it with organic copper fungicides.
Also called early blight, this fungus affects hundreds of plants worldwide. Like other fungal diseases, it spreads quickly and easily when conditions are right and can go dormant, striking again when conditions are ideal.
Scientific Name: Alternaria spp., specifically Alternaria zinniae
Causes: Alternaria zinniae is a specific Alternaria strain that is most common on zinnias, but other Alternaria strains can sometimes cause blight symptoms.
Symptoms: These will start as small reddish-brown spots with gray centers that will become larger, overlapping, and may affect stems and petals if untreated.
Solutions:
- Organic copper fungicides may delay a spread if sprayed at the first sign of detection and then every seven to ten days following for the remainder of the season. Remove any already-infected plant tissue before spraying.
- Powdered sulfur can help stop fungal propagation. However, do not combine the use of copper and sulfur; leave at least two weeks between product applications.
- Remove all plant debris from your garden throughout the year after pruning, deadheading, and harvesting, as well as before winter.
- Practice proper crop rotation.
- Select disease-resistant cultivars.
- Clean tools thoroughly after each use.
- Deep root water rather than overhead watering.
Bacterial Diseases
Bacterial Leaf Spot
Xanthomonas thrives when there is prolonged wet and warm weather.
This bacteria thrives in prolonged wet and warm weather (above 70° at night is ideal) and when relative humidity is 85% or above. At least three weeks of low humidity and lower temps can halt the disease altogether, even if ideal conditions present later.
Scientific Name: Xanthomonas
Causes: Xanthomonas campestris pv. zinniae
Symptoms: Early symptoms can include water-soaked leaf spots, which may dry up eventually and cause leaf drop. Spots may become larger and dark purple or brownish, developing a yellow halo. The plant has a tattered appearance.
Solutions:
- Select disease-resistant cultivars. If you buy transplants from a supplier, be sure you know about their sanitary practices so you don’t unknowingly bring the disease to your garden.
- If you suspect the inoculum is present on seed surfaces, you can wash them in a solution of eight parts water and two parts bleach for 40 minutes. However, any disease inside the seed will not be affected.
- Copper fungicidal sprays may help stop the spread but don’t always work when it’s the only treatment used. Spray zinnias in bloom later in the evening.
- Remove all debris from gardens, as this bacteria can survive on debris for up to a year. Weeds and other host plants can act as host plants for Xanthomonas species, so it’s vital to move infected plants and debris to a burn pile.
- Practice proper crop rotation.
- Keep tools sanitized.
Other Diseases
Aster Yellows
Inform your local extension office and send plant samples to the diagnostics lab if you suspect aster yellows.
There is no cure for the disease known as aster yellows in any plant, so healthy cultural practices and detection are important. Report any suspected infestation to your local extension office after sending plant matter to the diagnostics lab to be tested and confirmed. Fortunately, this disease is not very common.
Scientific Name: Caused by Candidatus Phytoplasma asteris
Causes: A phytoplasma that lives within the vascular system of the plant or is carried to the plant by the aster leafhopper or another leafhopper species that hosts the pathogen. About two weeks after a leafhopper feeds on a plant infected with aster yellows, it will unknowingly inject a small amount of the phytoplasma into every other plant it feeds on for the rest of its life.
Symptoms: Yellow leaves and stems, stunted growth, misshapen and discolored flowers, or green flowers.
Solutions:
- Identify if you have aster leafhoppers or other leafhopper species if you suspect aster yellows. Populations are difficult to estimate as they are quick, and they can move in randomly with a disruptive weather system.
- Aster yellows can survive in the crown and roots of infected plants but not the debris. It’s important to remove debris and burn it.
- Do not use infected garlic cloves as seed garlic, or you will continue to infect future crops.
- Insect netting may prevent leafhoppers carrying the pathogen from reaching your zinnias.
- Reflective mulches and plastics may disorient leafhoppers and keep them away.
- Neem oil can decrease leafhopper populations but may not prevent aster yellows from spreading if a host for the phytoplasma is already on the plant.
- Control perennial weeds that may harbor the pathogen.
Foliar Nematodes
Infected plants should be promptly and completely removed and destroyed in a burn pile.
Harmful nematodes can live on and inside around 450 plant species, usually affecting hostas but also infecting other herbaceous plants, woody ornamentals, and flowers. Don’t confuse them with beneficial nematodes, which can be your partners in reducing soil pests.
Scientific Name: Aphelenchoides spp
Causes: Worm-like organisms of the genus Aphelenchoides, not visible to the naked human eye.
Symptoms: Young plants are particularly at risk for stunted growth, leaf curl, browned, dry leaves that may fall off, and blotchiness. Foliar damage is usually just cosmetic.
Solutions:
- Remove infected plants fully and immediately. Destroy them in a burn pile.
- Opt for drip irrigation as opposed to overhead watering.
- There are no known cures for foliar nematodes. You can inspect plants brought in, but they will not always show symptoms at early growth stages, so it can be tough to spot before it’s too late.
Note: Hot-water treatments have been used to treat rare, high-value plants. However, these methods are not recommended for home gardeners as a safe or sustainable option. Hot water treatments require submerging all foliage in hot water at 50º C / 112º F for 5-10 minutes, which can cause damage and wilting to the foliage; this is a technique we do not recommend trying at home.
Pests
Japanese beetles
To address this issue, you can set up Japanese beetle traps in your garden.
Japanese beetles can be extremely damaging to a zinnia crop, especially when newly sown or transplanted when they are most vulnerable. This iridescent black beetle releases a pheromone, attracting other beetles to come to the area and mate. They reproduce quickly and can destroy a zinnia patch in a short amount of time if left unnoticed.
Scientific Name: Popillia japonica
Symptoms: Skeletonization of foliage
Solutions:
- Japanese beetle traps can be installed around your garden. Just empty the traps and replace the lure bait to ensure consistent effectiveness. Place them away from other valuable crops so they aren’t attracted to those instead.
- Plant trap crops like marigolds and nasturtium throughout and along the borders of zinnia beds.
- Handpick early morning as soon as infestations are noticed as they grow quickly. Deposit the beetles into a jar full of soapy water. Do this often to keep the populations under control.
- Applications of beneficial nematodes to the soil, particularly Steinernema feltiae, can be used as preventatives in the fall. These beneficial nematodes attack and kill overwintering Japanese beetle grubs in the soil.
Lacebugs
These bugs pierce ornamental trees and leaves from the underside, extracting the plant’s juices.
Lacebugs pierce the underside of ornamental trees and leaves, sucking the plant’s juice out. This damage will not just make zinnias less attractive and less marketable but also leave them more vulnerable to future diseases and pests.
Scientific Name: Z. angustifolia
Symptoms: Tiny pale dots and bleaching throughout the foliage. A fierce infestation can cause damage to be visible atop the leaves versus just the underside.
Solutions:
- Insecticidal soaps can be sprayed on all surfaces of the plant. Lacebugs will die after consuming the soap.
- Spinosad sprays can be used according to instructions. This is an organically approved bacteria used worldwide to control all kinds of pests.
Asiatic Garden Beetle
There are no known natural predators or effective biological control methods for these beetles.
These slightly iridescent orangish brown beetles hide in your soil during the day, waiting until nighttime to fly and wreak havoc in the garden. They may feed on roots underground but mostly will feed on the foliage of a large range of plants. Growers in the Northeast experience just one generation, whereas warmer regions may experience more than one.
Scientific Name: Maladera castanea
Symptoms: Some people consider damage from the Asiatic garden beetle (AGB) only a nuisance, but they can destroy leaves off a patch of zinnias if populations are out of control. Their damage often goes undetected because they only feed at night, and other beetles are blamed for their damage.
Solutions:
- Spraying zinnias with neem oil may prevent beetles from feeding on them or may kill those that do feed on them.
- Handpick in the same fashion as listed under Japanese beetles. Only do so at night when they are active.
- Unfortunately, there are no known natural predators for this beetle and no known effective biological control methods.
- Chemicals can be used to control AGB grubs in certain settings. When using chemical control, read all instructions thoroughly and take caution around pets and children.
Pro tip: If you can’t seem to find an active pest during the day, but you’re seeing some foliar damage, you should suspect this beetle as the culprit if it’s common in your area. Do some scouting at night to confirm.
Aphids
Detecting aphids is easy as they remain on the plant and cause discoloration on the underside of leaves.
These tiny green or white bugs can get out of control fast, and their damage can be costly. They pierce holes in the leaves and suck moisture out of the plant, leaving zinnias wilted and discolored. Luckily, aphids are easy to spot.
Scientific Name: Aphidoidea
Symptoms: Tiny puncture wounds and discoloration on the underside of the leaves. Visible aphids may be on the undersides of leaves or on the stems.
Solutions:
- Plant herbs and other flowers nearby that attract beneficial insects like yarrow, dill, mint, and fennel.
- Plant alliums nearby that should deter aphids naturally. Anything extremely aromatic should drive them away.
- Cultivate healthy and well-balanced soil and attract beneficial insects and natural predators of aphids.
Stalk Borers
To prevent moths from laying their eggs on your zinnias, consider using insect netting to keep them at bay.
There is only one generation of this pest, but it can cause significant damage if not caught. Moths come out to lay eggs in late summer or early fall. Eggs overwinter and hatch in April and May. The adult of the stalk borer is a whitish-brown caterpillar with a purple band around the center and three distinct brown stripes.
Scientific Name: Papaipema nebris
Symptoms: Wilting and death. Larvae tunnel through stalks of zinnias, consuming the innards as they go. Eventually, the top wilts and falls over, causing death. Often, it’s too late once the damage is noticed.
Solutions:
- Prevent moths from laying eggs by covering zinnias with insect netting.
- Sprinkle diatomaceous earth at the base of plants and on foliage. This may deter them.
- Keep gardens clear of debris and weeds, as the caterpillars may overwinter here.
- Burn any affected stems in the fall.
- Scout grass cover crops such as rye and wheat for eggs in the fall and spring.
Final Thoughts
Zinnias are a great beginner flower and make any landscape or garden cheerful and bright. Be sure not to over or underwater, know what type of pest damage to look for and how to control the culprits, and take care to prevent and control disease.
