Your Cool Home is supported by its readers. Please assume all links are affiliate links. If you purchase something from one of our links, we make a small commission from Amazon. Thank you!
Basil leaves turn purple for a few different reasons, and it’s usually a sign that your plant is under some kind of stress.
If you’ve noticed that purple color creeping into your basil, you’re not alone, and it doesn’t automatically mean your plant is doomed.
The most common culprit is a phosphorus deficiency in the soil, but you might also be dealing with cold temperatures, or even just a natural genetic quirk of the variety you planted.
In this post, we’re going to dig into why your basil leaves are turning purple, how to fix each cause, and what you can do to prevent it from happening again.
Let’s get to the bottom of this purple mystery together.
Why Are My Basil Leaves Turning Purple?
So, why are your basil leaves turning purple?
It all comes down to a pigment called anthocyanin.
Every basil plant produces this pigment to some degree, but when the plant gets stressed, it starts churning out way more of it.
That excess anthocyanin shows up as purple or reddish blotches on the leaves, stems, and even the veins.
Let’s break down the main reasons why your basil leaves are turning purple and how to identify which one you’re dealing with.
1. A Phosphorus Deficiency in the Soil
The number one reason basil leaves turn purple is a lack of phosphorus.
Phosphorus is a vital nutrient for basil because it helps with energy transfer, root development, and photosynthesis.
When your basil can’t get enough phosphorus, it can’t complete those processes efficiently.
The plant responds by creating more anthocyanin, which makes the leaves turn purple.
You’ll often see this start on the older, lower leaves first because the plant moves whatever phosphorus it has to the newer growth at the top.
A soil test will tell you for sure if phosphorus levels are low, but you can also spot this by checking if the purple is concentrated more on the underside of the leaves.
To fix this, you’ll want to add a fertilizer that’s higher in phosphorus than nitrogen or potassium.
Look for a fertilizer with a higher middle number in the N-P-K ratio, like a 5-10-5 or something similar.
Apply it according to the package instructions and your basil should start greening up within a week or two.
2. Cold Temperatures and Chilly Nights
Another super common reason basil leaves turn purple is exposure to cold.
Basil is a tropical plant that absolutely hates the cold.
When temperatures drop below 50 degrees Fahrenheit, the plant’s metabolism slows way down.
This slowdown makes it harder for the basil to take up nutrients, especially phosphorus, even if there’s plenty in the soil.
So, even with a perfect soil nutrient profile, cold roots and cold air can trigger that same purple pigment response.
If you’ve had a cold snap or if you’re growing basil outdoors in early spring or fall, this is very likely the cause.
You’ll notice the purple often appears quite suddenly after a chilly night.
The fix is simple: bring your basil somewhere warmer.
If it’s in the ground, you can cover it with a frost blanket at night.
If it’s in a pot, move it inside near a warm, sunny window.
Once the temperatures warm back up, the purple often fades and the new growth will come in green.
3. You’re Just Growing a Purple Basil Variety
Now, before you start scrambling to fix a problem, check what kind of basil you’re actually growing.
There are some basil varieties that naturally have purple leaves as their signature look.
Varieties like Dark Opal, Purple Ruffles, and Red Rubin are all bred specifically to be purple.
If you planted one of these, your basil leaves are turning purple simply because that’s what they’re supposed to do.
This isn’t a sign of stress or deficiency at all.
These varieties have high natural levels of anthocyanin, and they taste wonderful in pesto, salads, and vinegars.
So, you can just relax and enjoy the gorgeous color and unique flavor.
4. Root-Bound Plants and Poor Drainage
If your basil is in a pot that’s too small, the roots might have filled up the entire container.
When basil becomes root-bound, it can’t take up water and nutrients effectively.
This leads to a nutrient deficiency, and guess what that looks like?
You got it: purple leaves.
Similarly, poorly draining soil can cause root rot and damage the root system.
Damaged roots can’t absorb phosphorus, and that will also trigger the purple.
Check if roots are poking out of the bottom drainage holes or if the soil stays soggy after watering.
If so, it’s time to repot your basil into a larger container with fresh, well-draining potting mix.
And always make sure your pot has holes at the bottom so excess water can escape.
5. Overwatering and Stressed Roots
Speaking of water, overwatering is a sneaky way to get purple basil leaves.
Basil likes consistently moist soil, but it doesn’t like to sit in a puddle.
Soggy soil chokes out the roots by displacing oxygen, which the roots need to breathe and take up nutrients.
When the roots can’t do their job because they’re waterlogged, they can’t absorb phosphorus efficiently.
And again, a phosphorus deficiency will make your basil leaves turn purple.
The solution is to let the top inch of soil dry out between waterings.
Stick your finger in the soil, and if it feels dry, it’s time to water.
If it’s still damp, wait another day or two.
Water deeply but less frequently to encourage strong, healthy root growth.
How to Fix Purple Basil Leaves
Now that you know the common causes, let’s talk about the practical steps you can take to get your basil back to its vibrant green self.
1. Start with a Soil Test
Before you dump a bunch of fertilizer on your plant, it’s a smart idea to check your soil’s pH and nutrient levels.
A simple soil test kit from your local garden center can tell you if phosphorus is actually the problem.
Basil prefers a slightly acidic to neutral soil pH, around 6.0 to 7.0.
If the pH is off, your plant might not be able to absorb nutrients, even if they are present in the soil.
Correcting the pH to the right range can often solve the nutrient uptake issue and stop the purple.
2. Feed Your Basil the Right Fertilizer
If you confirm it’s a phosphorus issue, apply a balanced, water-soluble fertilizer.
Look for one with a mid-range phosphorus number, like a 10-20-10 or a 5-10-5.
Mix it at half strength and feed your basil every two to four weeks during the growing season.
Make sure the soil is moist before you fertilize so you don’t burn the roots.
Liquid kelp or fish emulsion are also excellent organic options that many gardeners swear by for fixing purple leaves.
3. Provide Consistent Warmth
To prevent cold stress, make sure your basil is in a location that stays warm and sheltered.
If you’re growing indoors, keep it away from drafty windows and air conditioning vents.
Outdoors, plant basil only after all danger of frost has passed and the soil has warmed up.
If you need to leave it outside on a chilly night, consider covering it with a cloche or a simple plastic jug with the bottom cut out.
4. Water the Right Way
Always water your basil at the base of the plant, not from overhead.
This keeps the leaves dry and prevents fungal diseases.
Water early in the morning so the leaves have time to dry out before the cooler evening temperatures arrive.
And use your finger to check the soil moisture before you water.
So, Why Are My Basil Leaves Turning Purple?
Your basil leaves are turning purple because the plant is producing extra anthocyanin in response to stress.
The stress is most commonly from a phosphorus deficiency, cold temperatures, or a root problem like overwatering.
You can fix purple basil leaves by adjusting your watering habits, feeding with a phosphorus-rich fertilizer, and keeping the plant in warm conditions.
But also don’t forget that some varieties just naturally have purple leaves.
Once you’ve identified the cause and made the right adjustments, your basil should bounce back and start pushing out healthy green leaves again.
The most important thing is to check your growing conditions and make the necessary tweaks.
Happy growing and enjoy your delicious, healthy basil!