Our garden is the farm's pride and joy. We love spending time in it, and preparing meals out of our fresh produce.
We add zucchinis to nearly we everything eat: soups, salads, stir fries, baking, and smoothies. If you eat zucchini as much as we do, then you know how important it is to grow them in the garden and have a good harvest. Sometimes, however, the perfect harvest is blighted by yellow leaves and sick plants.
Zucchinis are very susceptible to many issues that can cause the leaves to turn yellow. If ignored, one yellow leaf can lead to the loss of your harvest and ultimately the death of the plant. It is important to be able to identify what is causing the yellowing leaves. Then you can naturally correct the problem and (hopefully) nurture your plant back to health.
Why Do Zucchini Plants Turn Yellow?
Like all plants, zucchinis produce chlorophyll. Chlorophyll is the pigment that colours a plant green and makes photosynthesis possible. Chlorosis is when the leaves of the plant turn yellow due to a loss of chlorophyll. Chlorotic leaves can be a big issue for a plant: not only are they unsightly, but they are very susceptible to diseases and insect infestations, and they are easily scorched by the summer sun.
When the leaves of a plant turn yellow, this is usually a sign that there is a more serious issue present in your garden.
Here are 7 issues that arise in the garden that will cause your zucchini leaves to turn yellow. Let’s look at how to solve these issues naturally.
- Nutrient deficiencies and pH
- Overwatering and underwatering
- Damaged roots
- Lack of sunlight
- Transplant shock
1. Nutrient Deficiencies and pH
One of the most common causes of chlorosis is nutrient deficiencies. There are several nutrients whose absence can cause yellow leaves, and a slight imbalance in the chemical composition of the plant or soil can also lead to chlorosis. The best way to know if you have a nutrient deficiency is to have your soil tested at a local lab. Once you have determined which chemical is to blame, you can set about rectifying the issue.
Nitrogen helps promote green, vegetative growth in your zucchini plants. A common cause of yellow leaves is a lack of nitrogen. Nitrogen deficiency can often be told apart from other mineral deficiencies because it will affect the oldest, or innermost leaves first before causing the new growth to turn yellow.
Iron is essential for chlorophyll production. A plant only needs a small amount of iron to properly grow but the unavailability of iron can quickly lead to yellowing of the leaves. Iron chlorosis can usually be identified as it will affect the younger, outermost leaves first before moving to the older leaves in the middle of the plant.
If your soil is lacking in iron, you can simply add some to the soil which will then be absorbed by the plant. Greensand is a great, environmentally-friendly option that can add iron to your soil. It is made of iron oxide which will not only add iron to the soil but can help loosen clay soils (which will help with drainage as we will learn later). Ask your local garden centre for other natural, eco ways to improve the iron in your soil.
Other Nutrient Imbalances
Your soil might actually have enough iron in it, yet your plants can still suffer from iron chlorosis. This is because many other nutrients will tie iron in the soil and block its absorption by the plant. Too much calcium, manganese, phosphorous, copper, or zinc in the soil can bind the iron so the plant cannot use it. Alternatively, too much potassium in the plant will reduce the amount of iron it can use.
In many cases, the main culprit is improper use of fertilizer. If you use a fertilizer, make sure it is low in phosphorus (the middle number in the N-P-K ratio). Also, if you are faced with iron issues, avoid using cow manure to fertilize your soil since cow manure is generally higher in phosphorous than nitrogen and can aggravate any issues you have.
How to Solve Nutrient Imbalance in the Soil
The most natural solution to correct nutrient imbalances is to add compost or well-rotted manure (except for cow manure, of course). Compost will add potassium and phosphorous to your soil so it can be slowly absorbed by the plants without creating an imbalance. Adding well-rotted manures, especially from your hen house or horse stable, will add leaf-building nitrogen.
Compost and manure will also build and enrich your soil with humus. Humus is the rich, sweet-smelling final product of composting organic matter. Humus will add bulk to the soil and dilute any over-abundant nutrients. It will help disperse the nutrients and allow excess to wash away, and ultimately bring the soil into balance.
The ideal soil pH for your zucchinis is between 6.5 and 7.0. Any higher, and your soil will become alkaline and this can be a contributing factor to chlorosis. Be particularly careful of this if you add lime to your soil.
Again, the best way to fix an imbalanced soil pH is to add compost. If in doubt, you can never go wrong by adding lots of compost and well-rotted manure.
2. Overwatering and Underwatering
Another very common cause of yellow zucchini leaves is giving the plants either too much or too little water. Plants draw nutrient-laden water in through their roots. The water is spread throughout the plant where the nutrients feed the stem and leaves before the water is evaporated into the atmosphere in a process known as transpiration. Furthermore, most plants are made of 80% to 90% water so they need to have enough moisture available to thrive.
Ironically, both overwatering and underwatering can lead to your plant not getting enough water.
If you give your plants too much water, the soil becomes saturated and the roots do not have access to oxygen which it needs to grow new roots. This means the growing plant above ground is supported by insufficient, stunted roots. Since the roots are not able to uptake enough nutrients, so the plant is not able to make chloroplasts which leads to chlorosis. In short, the plant is drowning.
Zucchini plants don’t need as much water as you might think. About 2 cm to 3 cm (1 inch) of water per week is generally sufficient. In our garden, our zucchinis have always grown very well with rainfall alone. In the last decade, we only watered our zucchinis during one particularly dry summer. The rest of the time they produced beautifully without our intervention.
If and when you water, make sure you don’t water the leaves as this can lead to fungal diseases (see below). A drip irrigation system is a great way to apply water directly to the soil.
It is important to know what kind of soil you have and take extra care watering when you have heavy, clay soil. Soil particles in clay are very small and so pack very close together. The tightly compacted soil traps water and does not let excess drain away, leading to waterlogged soil and drowning roots. If you have clay soil, again the solution is to add compost which will help loosen the soil and improve drainage.
If your leaves are turning yellow from overwatering, the best solution is to wait until the soil dries, and then begin watering again, but this time in moderation.
When your zucchinis do not get enough water, the plants will start to dry and the withered leaves will often turn yellow. Since there is not enough water in the soil, the plant cannot take on nutrients via the roots and chlorophyll cannot be adequately produced.
If the zucchini leaves are yellowing from dehydration, start watering your plot. While it may be tempting to dump lots of water on your thirsty plants, this can lead to overwatering and start a whole new problem. Always water in moderation, about 2 cm to 3 cm (1 inch) per week.
3. Damaged Roots
If the roots of your zucchini plant become damaged, this can also cause the leaves to turn yellow. Just like with overwatered plants, the damaged roots are not able to absorb enough nutrients to feed the plant and will not produce sufficient chlorophyll.
If you have recently moved or transplanted your zucchinis, or if you dug close to the base of the plant, this might be the cause of the yellowing leaves. Unfortunately, there isn’t much you can do about this, except try and avoid further stressing the plant until the roots recover.
4. Lack of Sunlight
Sunlight is essential for photosynthesis, and this goes hand in hand with chlorophyll. Zucchini need 6 to 8 hours of sun a day, and a lack of sunlight can cause the leaves to yellow. If your zucchinis are planted in a shady spot of the garden, there, unfortunately, isn’t much you can do about this.
Potted zucchini can be carried to a sunnier location, while plants growing indoors or in a greenhouse will benefit from artificial light.
5. Transplant Shock
If you start your zucchini indoors, they can sometimes turn yellow when they are transplanted into the garden. This is known as transplant shock. The plant is simply adjusting to its new environment and usually takes a few days to adjust and bounce back.
To minimize transplant shock, make sure your zucchini transplants are fully hardened off before you put them in the garden. Here is a good article about eco-friendly ways to reduce transplant shock to new transplants.
Most of the time, your zucchinis will grow strong healthy plants with a bountiful crop. There are a plethora of diseases that affect zucchinis that will cause the leaves to turn yellow. To prevent diseases in your garden, your best defense is crop rotation. When you grow the same plant in the same spot year after year, diseases and sicknesses can last in the soil from the fall and infect your zucchinis again in the spring. Moving your zucchinis to a different spot each year will break this cycle. Ideally, you don’t want to plant zucchinis in the same spot for three or four years.
Here are the 3 most common diseases that cause yellow leaves and how to treat them.
Cucumber Mosaic Virus
Despite its name, the cucumber mosaic virus will affect all squash and is a common issue with zucchinis. Infected plants will have splotchy yellow leaves, and the zucchinis themselves will be stunted, poor-tasting with yellow spots as well.
There is no cure for the cucumber mosaic virus, and infected plants should be pulled and disposed of (but do not put them in your compost). This virus is spread by aphids so your best bet is to deter and eliminate aphids before they bring the disease. See below for how to deal with “pests” in the garden.
Fusarium wilt also causes zucchinis’ leaves to yellow. It is a fungus whose spores can survive over winter and is spread by the cucumber beetle. Again, remove the diseased plant, practice strict crop rotation, and eliminate the bad bugs.
Is closely related to algae, and so thrives in cool, damp areas. There will be fuzzy fungal spores on the underside of the leaves, and the leaves can have yellow spots among other colours. The spores spread leaf to leaf, and plant to plant, in the wind, and can live for years in the soil.
Downy mildew is often not fatal and can disappear when the weather warms and conditions dry out.
Your best defenses to stave off downy mildew are to pick off any infected leaves or remove whole plants if necessary. Thinning your plants will also help to allow air circulation and sunlight to warm and dry the plants and surrounding soil. Also, a long crop rotation is very important.
Sometimes your zucchini plants will become the home of several different insects. Some insects will eat large holes in the leaves causing them to wilt and yellow. Other insects will suck sap from the leaves of a zucchini plant, draining nutrients and causing the leaves to yellow. Learning how to identify which bugs you have will let you know what to do to save your plants.
Here are the main types of bugs that you are most likely to be dealing with.
As we mentioned earlier, aphids are spreaders of cucumber mosaic virus. On top of that, they suck sap from the leaves. They can often be identified by a black sticky residue on the bottom of the leaves.
Spider Mites will drill small holes in the leaves and suck sap and drain nutrients. You can tell if you have spider mites by the webs they leave behind on the leaves.
Squash bugs also eat the sap from the leaves and leave yellow spots that will often turn brown. They can often kill young plants and reduce the yield of mature plants.
Squash Vine Borers
These bugs also love to feed on zucchini and can cause significant damage to the plant, and causing the leaves to yellow. A good solution to these bugs is to pick them off by hand.
How to Deal With Insect Infestations
The best way to deal with problematic insects is to attract beneficial insects that will prey on the bad bugs. This can be best achieved by companion planting in the garden. Companion planting is growing other plants that benefit your zucchinis. Most pollinators are also predator insects, so growing plants that flower and attract pollinators is a great and natural way to reduce bad bugs or to eliminate them altogether.
Putting in shallow water dishes with a protruding rock will create a watering hole for good bugs and will attract them to stay around and eat more of the bag ones.
Another aspect of companion planting is to use other plants to repel insects that you don’t want. Onions, or other plants of the Allium family, will repel a lot of undesirable insects with their pungent odor.
Floating row covers can also be used to naturally keep insects away from your zucchinis. These are fine mesh sheets that are laid over your plants that keep bugs from landing on the plants If you choose to use floating row covers, make sure you remove them as flowers form so pollinators can access the flowers.
Dealing With the Issue Effectively and Naturally
There is nothing as frustrating as watching your carefully cultivated plants yellow and die before their bounty can be harvested. Whether your plants are infested with bugs, affected by disease, or suffering from a lack of iron, I hope this article gives you enough information to assess why your leaves are turning yellow so you can deal with the issue effectively and naturally.
This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.
© 2021 Bellwether Farming