Bitter Basil – Why and What to Do With It

You have proudly grown your basil and, at the moment of harvest, you end up with an unpleasant surprise. The leaves, that perhaps you added on top of your pasta, taste bitter. You might wonder what’s happened, whether this is normal and what can you do to avoid such a problem in the future. Here is a bit of clarification on the subject.

As a general rule, basil leaves can become bitter because of 1) flowering, 2) inadequate pruning, 3) natural chemical components, and 4) specific variety or cultivar. However, there are ways to lessen or prevent the basil from becoming bitter. Nevertheless, bitter basil leaves are still edible and can be used for cooking.

Hence, how can you prevent such from happening? Let’s keep reading!

1. Flowering Makes Basil Bitter

Most types of basil are annual plants, which means that, at most, they last up to a year. At the end of its lifecycle, the plant starts producing flowers that, will then originate new seeds. This represents the final purpose of the plant that will then die as biologically programmed to do so. 

During such stage, the plant will use its last energy in developing flowers and seeds leaving fewer resources for everything else, leaves including. Indeed, leaf production will stop and the leaves already developed will experience a drop in the natural oil content causing a bitter taste to prevail.

Hence, the question is: can such natural phenomena be delayed? As strange as it might sound the answer is yes!!

2. Inadequate Pruning Makes Basil Bitter

Indeed, basil is fighting to capture as many resources as possible to develop and produce flowers and through them its future generation through seeds.

But if you harvest your potted basil regularly to prevent it from blooming, or take away the flower straight after they appear, the plant will not have time to produce fully complete flowers and seeds. This is key! 

Although the plant will last longer following this approach, it will not last forever. After a full year, it will start dying. However, regular pruning allows your basil to produce way more leaves that never got bitter as the plant will never have the time to focus its energy on producing flowers and seeds.

3. Natural Chemical Components Make Basil Bitter

The presence and concentration of over 40 chemical components in basil, and herbs in general, can naturally lend it a bitter flavor profile. External factors can affect these compounds and further enhance the bitter taste of basil. 

People tend to overlook the fact that the whole basil plant is edible. You can eat its leaves, stems, and flowers no matter how bitter they can get. But why exactly do they taste like that? Does that only happen when gardeners make a mistake while cultivating basil?

In reality, basil—regardless of its specific variety—contains tons of flavor compounds that may have a bit or a lot of bitterness to them. To date, 45 chemical compounds that are known to have a bitter flavor profile have been identified in basil [1, 2]. 

More specifically, such compounds can be grouped into terpenes, phenylpropanoids (or flavonoids) and polyphenols, and fatty acids as well as their derivatives (e.g., aldehydes, ketones). 

Chemical Type	Basil Compound	Basil Flavor
Terpenes	Alpha-Terpinene	Bitter Citrusy Herbal Lemon Medicinal Terpene Woody
Terpenes	Gamma-Terpinene	Bitter Gasoline Herbal Citrusy Oily Tropical Terpene Turpentine Woody
Phenylpropanoids Polyphenols	2-Phenylethanol	Bitter Floral Honey Spicy
Phenylpropanoids Polyphenols	Phenylacetaldehyde	Bitter Clover Cocoa Floral Grapefruit Green Hawthorn Honey Peanut Sweet
Fatty Acids Derivatives	3,4-Dihydroxybenzaldehyde	Almond Bitter Dry Medical
Fatty Acids Derivatives	4-Methoxybenzaldehyde	Almond Anise Balsam Berry Bitter Cherry Chocolate Cinnamon Creamy Floral Hawthorn Mimosa Minty Powdery Vanilla Sweet
Fatty Acids Derivatives	Benzaldehyde	Almond Bitter Burnt sugar Cherry Sharp Strong Sweet

As seen in the table above, these compounds naturally impart some bitterness to basil, among other flavors.

4. Specific Varieties and Cultivars Make Basil Bitter

Certain basil varieties or cultivars, such as the holy basil (Ocimum tenuiflorum), inherently have a more prominent bitter flavor profile than others. This is because of the differences in the concentration and composition of their chemical components.

Whether cultivated intentionally or just naturally obtained, you can expect different types of basil plants to have varying flavors. Depending on what’s available in your local markets, you may or may not have bought bitter basil.

Holy basil, for instance, despite being notably bitter and spicy compared to say, sweet basil (Ocimum basilicum) is commonly used in India—where it originates from. Because of that, people there have become accustomed to its bitterness and enjoy using it in cooking.

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If you grew up on sweet basil, you probably won’t be able to appreciate holy basil as much. Because in comparison—and as its name suggests—sweet basil is naturally much sweeter than other kinds due to compounds like linalool and eugenol [3].

So, yes—basil varieties and/or cultivars have very specific flavor profiles based on the types of compounds it has and how concentrated they are. 

Basil can be cultivated to be more sweet, citrusy, or whatever you want it to taste! Given, of course, that you have knowledge when it comes to food science. This is why some experts put a lot of time and effort into developing newer and better-tasting basil plants.

Can You Use Bitter Leaves For Pesto?

Perhaps you have forgotten to prune your plant in time and now you are with a bunch of bitter basil leaves that you do not want going to waste. Can you still prepare a bunch of good pesto to share with family and friends?

Although I do not recommend using bitter basil leaves for pesto as the flavor will be affected there are a few ways to reduce its bitterness such as 1) blanching, 2) mixing it with other herbs and leafy greens, 3) changing the recipe’s ingredient ratios, and 4) adding some sugar.

1. Blanching

Following the approach discussed in one of my previous articles here, this technique allows you to drastically increase the life of your vegetables and herbs once frozen reducing significantly the rate to which texture and flavor are lost. Moreover, as a side advantage, reduce the bitterness of the basil.

2. Mixing With Others

Mix with spinach, parsley, or non-bitter basil: you can double the amount of green you are using by adding to your bitter basil an equal amount in weight of good taste basil (you can buy it from the supermarket) or/and spinach/parsley.

3. Change Ingredient Ratios

Change the proportion of the non-basil ingredients: you can increase, compared to the original recipe you are following, up to 50% (depending on the bitterness of your basil) the amount of parmesan, and nuts so as to counterbalance the bitterness of the basil.

However, be careful as increasing the proportion of dry ingredients might make your pesto denser (so perhaps you might need a bit more oil or add a bit more boiling water to your pasta);

4. Adding Sugar

As some of you might know when preparing a tomato sauce, it is not uncommon to add 1-2 spoons of sugar. You can do the same here. I would start with a teaspoon, mix it with the other ingredients and then taste it and add more if necessary.

Can You Recover Bitter Basil by Starting Pruning Regularly?

Suppose that your basil developed quite a few flowers. If you start pruning it regularly preventing any other flowers to bloom are those bitter leaves going to get sweet again reversing the flowering process?

Unfortunately, if you left your basil flower and produce seeds, pruning will not bring your basil leaves to get sweet again. Once at that stage it is not possible to go back. You can reduce the bitterness of your leaves by cutting them from the plant and consuming (or preserving) them as soon as you can.

This will not give time for the leaves to totally lose their flavor. Moreover, your basil once developed the first seeds is going to die relatively soon as you can notice also from its stems becoming woody.

However, if the flower just developed, and you cut your basil back to the first sets of leaves (cut the basil on the steam that is above the first couple of leaves starting from the soil) then it is quite likely that the new leaves will be as tasty as before (or way less bitter).

This strongly depends on how early you managed to remove the flower that starts developing, the earlier the higher possibility to have tasty leaves again.

2 Tips for Pruning Basils

Tip #1: if you are leaving for a few days (for vacation for instance), prune your basil aggressively (for instance at half of its height) to make sure it will not have the time to flower while you are away.

Tip #2: even if you are not going on a vacation do not be afraid to prune aggressively your basil (for instance halving its heights). This will improve growth and delay flower and so small bitter leaves compared to a more timid pruning.

Such aggressive pruning can be done 5 to 6 times during the life of your basil (an average of two times a month assuming a year of life in case of indoor basil that can survive over the winter season).

Another Use of Your Bitter Basil: a New Generation!

Another way to maximize the usefulness of your end-of-life basil is by trying to propagate it. However, if you did not manage to prune it in time to reverse the flowering process (lots of flowers on many different stems), I would suggest not attempting to propagate the plant by cutting.

Indeed, if the basil is already woody (one consequence of flowering), cuttings might not work.

In this case, it is way better to let the flower finalize its development, and get dry and brown. At this moment the seeds inside such flowers can be ready to be harvested and planted in a new pot to originate new fresh basil plants. 

To harvest such seeds simply shred with your finger the dry flowers and that will easily turn into crumbles. The seeds will be easy to identify as the only hard part inside the flower, dark/black and all with similar shapes (often oval).

Then I do recommend, if you are not planting the seed straight away, to save them in a letter envelope or even better in a small plastic air-tight container that can be closed and left in a dry and dark place. A few gardeners also place the seeds in the freezer for a few days in an attempt to kill any small insects or eggs that can be on the seeds. 

On the other hand, if you did not give time for the plant to flower in the hope to produce more leaves and reverse the flowering (and bittering) process cutting can still be an option to propagate your basil and you should go for it. 

Tip: if you are undecided if your basil is in the first or second stage, I would go for both options! I would do cutting in one/two stems to see if it propagates while leaving the flower to develop in the original basil. This will reduce a bit the number of leaves you might harvest but that should not impact significantly as your plant is at the end of its life anyway.

Related Questions

Is it safe to eat bitter basil? Yes, the reduction in the oil content is altering the flavor but not the safety of eating the basil leaves

Are there basil varieties more bitter than others? Yes, some varieties of basil like the purple basil naturally present a more bitter taste compared to the most common “Genovese” one;

Can basil leaves get bitter because of cooking? Yes, basil leaves get bitter if cooked for too long. That’s also a reason why many basil recipes or usage of its leaves do not use cooked basil.

Sources

[1] “Optimising flavour in herbs” by Agriculture and Horticulture Development Board in AHDB Technical Review

[2] “Basil” by Indraprastha Institute of Information Technology Delhi (IIIT-Delhi) in FlavorDB

[3] “Basil: A Source of Essential Oils” by James E. Simon, James Quinn, and Renee G. Murray in Horticulture and Landscape Architecture|Purdue University

Further Readings

21 Tips to grow massive basil

Best container size for your basil

Best potting soil for your herbs

Why your basil has small leaves

Best basil fertilizer