Yellowing leaves of any plant is a symptom of chlorosis. That’s when you get yellow leaves but the veins retain their greenery.
The main reason it happens is a lack of iron. Petunias need iron to make chlorophyll. Without that, less sunlight is absorbed, and a direct result of that is the slowing of photosynthesis. The plant will still flower, but not as good as it would if it had the greenery of the leaves restored.
The problem with leaves on petunias turning yellow is that it isn’t always clear cut why it’s happening. It’s rarely just a case of increasing nitrogen, which is the primary nutrient responsible for the green pigment on plant leaves.
Sometimes there’s mottling, other times it’s yellowing veins, or just the tips of the leaves that’s turning yellow.
Each of the reasons why the leaves on petunias turn yellow have specific corresponding symptoms.
The 7 Possible Reasons for the Leaves on Your Petunias Turning Yellow
1 – Micronutrient Deficiencies
Several factors can result in petunias lacking the nutrients they need to keep their petals green and the flowers vibrant. It’s not all about the amount of fertilizer they receive. It’s also about what they share those nutrients with.
Speaking of fertilizer, you really need to be feeding these plants every month they’re in season. With thorough petunia care, these will put out an abundance of flowers. However, most of the energy they get from the nutrients in the soil is put into flowering.
Without adding nitrogen, the leaves will yellow. An easy tell of a nitrogen deficiency is when only the old leaves turn yellow but new leaves are green.
If you’ve already been fertilizing, don’t go rushing thinking it just needs a tad more, because a direct correlation to this problem is the soil the plant is grown in.
Soil needs to be neutral to slightly acidic for the plants roots to use the available nutrients. Alkaline soil binds up the nutrients making them insoluble, so your petunias won’t be able to use what you feed it.
Do a pH test of the soil and adjust if necessary. It should have an acidity of 6.0 to 7.0. If that’s all good, the next common problem to explore are deficiencies in micronutrients, which are mainly boron, iron, molybdenum, and zinc.
A visual inspection of the plant reveals which secondary nutrients are lacking.
The symptoms of specific nutrient deficiencies on petunias:
- Plants with a boron deficiency have yellowing happening at the tips of the leaves, and they’ll feel brittle.
- Plants with an iron deficiency have yellowing of the veins. This is referred to interveinal chlorosis. Chlorosis is when the entire leaf surface is yellowing. Interveinal chlorosis is when the leaf retains its greenery but the veins turn yellow. On new leaves though, the opposite happens when iron levels are too low. The leaves are yellow but the veins are green.
- Plants with a molybdenum deficiency have pale green leaves bordering on yellow, but will also be accompanied with brown spots on the leaves.
- Plants with a zinc deficiency have yellowing happening on old leaves at the base of the leaf first; not the tip of leaves.
The most common nutrient petunias struggle with is iron. Whichever symptoms your petunias are presenting with, the solution is likely switching the feed. It is probable that the fertilizer has an imbalance, such as too low a nitrogen concentration.
Each of the nutrients the plant needs should be available in the soil. If that’s within the pH range, all the nutrients will be soluble, meaning the plant can absorb them.
Another thing to consider relates to spacing, or to be more specific – overcrowding, which can easily happen with petunias in containers.
2 – Overcrowding, or Insufficient Spacing When Planted
Planting petunias too close together will result in compact roots. They’ll compete for the limited nutrients in the soil. The result will be several plants struggling.
As a guide for how many petunias to plant together, go with one plant per inch for mounding varieties, no higher than 20cm spacing for multiflora, and give trailing varieties up to 40cm space per plant.
If you are mixing plants, you’ll need to factor in the root size of companion plants. As an example, geraniums put down bigger roots (because it is a bigger plant) so they’ll need more space than say, Verbena; a trailing plant that puts out thinner rhizomes.
Always consider the size of the roots for every plant you include in a container. The bigger they are, the more nutrients they’ll consume. The bigger the plant, the bigger the roots will be.
In crowded containers and flower beds, the big guys can outcompete the smaller plants leaving your smallest with yellowing leaves and very little blooms.
3 – Too Much Water / Insufficient Drainage
Quick Tip: Overwatered petunias have yellow leaves all over. If the plant only has partial yellowing of the leaves, it’s something else.
Every plant has its preferences for watering. For petunias, much of the advice floating around states that they need regular watering to promote flowering. That advice is easy to misconstrue.
Overwatering isn’t just about watering your plants too much. It’s also how much hydration it gets from rainfall, and the type of petunias that’s showing the yellow leaves. There are various varieties, and different ones have different watering and soil requirements.
In flower beds, the soil can have a harder time draining. In potting mixes, it’s easier to tweak the mix (with perlite or vermiculite) to increase aeration and drainage as opposed to the soil in your garden flower beds.
As edging plants, Grandiflora varieties can take on too much water during a heavy spring shower. In areas with higher-than-average rainfall, there are petunia hybrids (of Multiflora and Grandiflora) that are tolerant of damp weather. There’s also Surfinia petunias, which are specifically hybridized for rain-resistance.
If you’re using petunias as a ground covering, the smaller variety of Floribunda are rain tolerant.
Consider the type of petunias that you’re growing, and the amount of water they are exposed to, if outdoors. A more appropriate variety may be better suited next season.
Petunias are annuals, so what you learn this season is to your advantage for next season. Like, despite being annuals, you can prolong their lifespan for a couple of seasons, because how long petunias last depends on whether you overwinter them indoors or leave them be in the garden. Petunias can’t survive a winter outdoors.
On the opposite end of the spectrum to overwatering, it’s also possible that your petunias aren’t getting hydrated enough!
4 – Dehydration
Again, this doesn’t reflect on you, necessarily. Higher temperatures and the growing conditions have a significant impact on the watering frequency petunias require.
Generally, petunias (of any variety) in hanging baskets need watered frequently. As often as three times weekly in the peak of summer. These aren’t exactly drought-tolerant.
Whether you have petunias in containers or in garden soil, they will always dry out fast in heat.
As the soil becomes drier, the leaves begin to wilt first, then the leaves yellow. As soon as wilting is evident, water your petunias.
The soil petunias are grown in should only be dry to the top couple of inches. Beyond that is going into the too dry territory. You may even notice the soil cracking on severely dehydrated petunias.
How you water petunias can impact their health too. Top watering (which is when take a watering can and pour it over the leaves and flowers) can leave the foliage wet for too long, encouraging pest infestations and, quite possibly, bacterial or fungal infections.
5 – Pest Invasions
Once you get up close inspecting your plant to see where the yellowing is progressing, don’t be surprised to see signs that something has been eating petunias.
A few pests are attracted to these, but more so when the soil is overly moist. Aphids are a common problem, and with overwatered petunias, fungus gnats are likely. The fungus or mold in and on the soil, and the abundance of moisture attracts fungus gnats. Fungicides and insecticides can get rid of both.
Asides from those, others types of feasting insects that leach the juices from the leaves include spider mites, thrips, and leaf miners, which cause petunia leaves to curl and turn yellow.
The solution for pests that feed on the juices in the leaves is to either knock them off when there’s only a few, and if there’s a lot, spray the plant down with neem oil or a similar insecticide.
6 – Diseases
Fungal diseases can be fatal to delicate annuals. Verticillium wilt and fusarium are soil-borne diseases that can wipe out delicate plants. As these are soil-borne, the lower portion of plants are inflicted first, starting with stem rot causing the lower leaves of petunias to turn yellow, shrivel, darken, then die. As these are annuals, there’s little point in trying to treat it with a fungicide, or replanting.
Any fungus growth in the soil will be detrimental, and is a main reason for petunias wilting and dying.
Another disease that has been known to effect petunias is mosaic virus, and it’s one that’s easy to identify. It causes the shape of the leaves to become distorted with a yellow mottling effect (yellow blending with the green) on the leaves.
7 – Lacking Sunlight
Due to the heavy flowering nature of petunias, they need lots of light. A minimal of 5-hours full sun is required. The brighter light they receive, the better they perform. 8-hours is optimal.
When petunias don’t get the light they need, photosynthesis is hindered. When that happens, leaves turn pale, and the flowering decreases.
For petunias in hanging baskets, they need to be rotated. Without turning them, one side will get all the sunshine, the others will wither away in the shade. The result will be lopsided growth, and an uneven distribution of leaves and flowers.
Full sun, all-round, is how to make petunias fuller, (and healthier), provided that they are having spent flowers pinched early too.
To get the full vibrancy of color, foliage, and blooms, turn baskets weekly or fortnightly for all the plants to get their fair share of full sun.
Growing up with a mom who filled her home (inside and out) with all sorts of plants, Lisa got her start in gardening at a young age. Living now on her own with a home and yard full of plants (including an indoor greenhouse), she shares all the gardening tips she’s gained over the years.