Why Your Basil Has Bitter Leaves? All You Need To Know

Sometimes basil leaves get bitter, there is a reason on why this is happening

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 a bit of clarification on the subject.

Why basil leaves get bitter? Basil leaves, independently from the species, change their original taste getting bitter when the plant flower. 

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

Flowers, Last Effort of Your Basil

Most types of basil are annual plants, that 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 flower and seeds leaving fewer resources to everything else, leaves including. Indeed, leaves 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!! Indeed, basil is fighting to capture as many resources as possible to develop and produce flowers and through them its future generation through seeds. However, if you harvest your potted basil regularly avoiding it to bloom or you 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 allow 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 flower and seeds.

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?

Bitter leaves and pesto, not a recommended combination
Photo by Jeremy Keith from Flickr

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: following the approach discussed in one of my previous article here, this technique allows 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. Mix with spinach, parsley or no-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 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, nuts so 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 adding a bit more of boiling water of 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 consume (or preserve) them as soon as you can. This will not give time to 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 steams in 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.

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

Tip #2: even if you are not going to a vacation do not be afraid to prune aggressively your basil (for instance halving its heights). This will improve growth and delaying 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 trying to propagate it. However, if you did not manage to prune it in time to reverse the flowering process (lots of flowers in many different stems), I would suggest to not attempt propagating 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 their development, 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 of 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 the attempt to kill any small insects, eggs that can be on the seeds. 

On the other hand, if you did not give time 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 just 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 you 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 in eating the basil leaves

Are there basil varieties more bitter than others? Yes, some variety 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.

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

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