Can You Use Fertilizer on Herbs? (How and When?)

When and how fertilizing herbs is crucial to understand how to make the most of them. But, should you fertilize them in the first place?

In general, herbs need to be fertilized due to the depletion of minerals in the soil over time. Fertilization should happen at the beginning of spring and its frequency depends on light conditions, soil quality, herb growth stage, and the herb’s health state. Indoor herbs might require more fertilizer than outdoor ones.

The truth is that you need to fertilize your herbs. However, how frequently? Definitely not as heavily as vegetables. How do you know when to start fertilizing? How to avoid the common overfertilizing mistake? Keep reading to find the answer to these questions and many more.

Fertilize Herbs in 3 Steps

Fertilizer for herb is necessary to rebuild the minerals content the potting soil loose after time, especially if in a container. These minerals are known as Nitrogen (N), Phosphorus (P), and Potassium (K).

Indeed, those are “nutrients” that your herb cannot produce by itself (from light, air, and water) and needs to take from the soil. Hence, the herbs need fertilizer when Nitrogen (N), Phosphorus (P), and Potassium (K) are missing. Each fertilizer is identified by (very often) 3 numbers known as N-P-K that determine the percentage of nitrogen, potassium, and phosphorus, respectively.

However, many beginner gardeners make the same number one mistake: overfertilizing. Follow these 3 steps approach to avoid disappointment:

  1. Step 1: Understand when to start fertilizing. Often later than you think. Here you can follow either measure the nutrients content, or start depending on the herb and potting soil “history”;
  2. Step 2: Decide what fertilizer you should use – strong fertilizer with high N-P-K values are often the worse fertilizer for herbs;
  3. Step 3: Define a fertilizer schedule – you might need to fertilize less than you imagine. This depends on your herb conditions.
Fertilize Herb Guide

Step 1: When To Start Fertilizing? 3 Cases

Herbs need fertilizer if they are in their mature stage (older than a few months) and the soil is depleted. This can be analyzed through an inexpensive soil tester.

If a soil tester is not available, then an herb should be fertilized depending on its history.

What does that mean? To decide when you start fertilizing, you need to have an idea of how you get that herb, how old if it has been fertilized before. More precisely, you might fall into one of these cases:

Supermarket Herbs

Herbs bought in small plastic containers from the supermarket need fertilizer usually straight away, especially if basil or parsley. It is not required for hardy herbs/shrubs like thyme, rosemary or lavender.

This is because the soil is pretty low in nutrients. Moreover, those herbs need a repotting anyway so I just change the soil usually. Have a look at this supermarket herb guide. The new soil should be a fresh potting mix. You can either do it yourself through this DIY potting soil guide or buy the best available in the market.

After the soil transplant with a fresh potting mix, you should start fertilizing after 6 months. Indeed, the majority of potting soils contain nutrients on them that will allow your herbs to thrive for many months to come without any external support.

Nursery

Herbs from the nursery do not require fertilizer for at least 6 months. The container is of the right size, and the potting mix is very likely to be of top-notch quality.

Seedling

Seedlings and seeds do not require fertilizer. Indeed, each seed has a package of nutrients (including the N-P-K minerals) that they use to develop. Applying fertilizer in seedlings will likely kill them due to the high salt content unsuitable for early root development.

When the seedling is ready to be transplanted in a larger pot, use a fresh, high-quality potting mix that will guarantee your herb to thrive for at least 6 months without fertilizer (case 1).

Step 2: Which Fertilizer Use for Herbs?

If you followed the previous steps, now the fertilizer should be easy to define.

Did you measure the nutrient content of your potting soil? Hence, in this case, you know what your potting mix is lacking. In this case, you might have two types of results:

  • All nutrients are missing more or less in an equal amount: your soil is deficient in nitrogen, phosphorus, and potassium more or less in the same way. In this case, you need a balanced fertilizer in which the content of the three nutrients is more or less the same. This is identified by the same N-P-K values on the fertilizer label. An excellent choice can be a mild 4-4-4 here on Amazon or a 10-10-10 if you find it.
  • One or two nutrients are less abundant than the others. In this case, you need an unbalanced fertilizer that supplies more of the missing nutrients and less of the ones you are OK with.

    For instance, in case your potting soil is missing nitrogen, then your fertilizer should be more abundant in this mineral (N, the first number in the N-P-K). For instance, for a mild fertilizer, a 6-4-4 as this one in Amazon is a good choice.

    In case phosphorus is deficient (P, the second letter in the N-P-K), then you need a phosphorus-heavier fertilizer. Here, for instance, you have good fertilizer from Amazon with a 2-8-4 level. A more extreme solution can be almost pure phosphorus, like this one found in Amazon. However, if you go for the pure version, use half or less of the suggested amount in the label. Indeed, the indication you have on the label might be likely referred for an outside herb that, in general, need way more fertilizer than potted ones.
  • Similarly, in case of potassium (K) deficient soil, go with a K-heavier N-P-K fertilizer. You might use pure potassium as this one sold on Amazon. However, this is something that, in general, I do not recommend due to the easy to make damage to your herbs (as very nutrient-dense). Again, go light with the dosage (half or less) to avoid problems.

What if you did not measure the nutrient content and you know that you need to fertilize anyway due to your herb history?

Well, in this case, expert gardeners and as well common sense do recommend a balanced fertilizer. This is a fertilizer with the same value for N-P-K. My best pick is a “mild” fertilizer (this minimizes the change to make a mistake) with a 4-4-4 concentration as this one on Amazon.

Also, you would not be wrong with more nitrogen/potassium fertilizer. Indeed, those two minerals are generally absorbed in a slightly faster rate by your herbs. Hence, a 24-8-16, for instance (or similar ratio), can be ideal. One of the most known and appreciated by gardeners is this one on Amazon.

Expert Tip 1: Chatting with many expert gardeners, they all agree that you do not need to use the amount suggested in the label. You can use half of what is recommended! Indeed, those label indications are for outdoor plants (or indoors with plenty of light) that need more fertilizer than the indoor version. Why? Because indoor herbs have a way lower metabolic rate (they consumed food faster) as they receive way less sun than their indoor counterpart.

Expert Tip 2: Check that the fertilizer (look at the label) is enriched with the so-called “micronutrients”. These are under the names of iron, calcium, sulfur, and magnesium. No worries about the amount, as in general, their content is really low (accordingly to what commonly herbs and plants need). They are absorbed in smaller amounts compared to N-P-K, but they are still vital for their development. Hence, if they are included, it is a great plus.

Step 3: How Often Should You Fertilize Herbs?

Fertilization is not an exact science, especially if you are not performing any measurement to understand how much or what you precisely need. However, there are a few reliable rules of thumb to decide how frequently you should fertilize.

The frequency of your fertilization depends mostly on the amount of light your herb is receiving. Is it summertime and your herb is receiving a sustained 6 to 8 hours of sunlight? I would go with 1 light fertilization (half of what is suggested in the fertilizer label) every 2 months. The same applies in winter in case your herb is growing in a temperature/light controlled environment (like a greenhouse, or a basement, as in this basement gardening).

What if your herb is going through low light and cold period? During cold and low-light winter days, many herbs enter the so-called “dormant stage” where their growth is arrested (or slowed down). Have a look at the best fertilizer for chives. Hence, during winter (in uncontrolled temperature and light environment), your indoor herb does not need any fertilizer. Applying it will only cause damage in the long term as not absorbed by the herb.

Hence, as a general rule of thumb you need to fertilize your grown potted herbs 3-4 times a year:

  1. None in winter: due to the dormant (or very low activity state), almost no minerals are required. This does not apply if you are using a grow light and temperature-controlled environment;
  2. Once (or Twice) in Spring: The herb is going a growing activity period, getting ready for the summer;
  3. Twice in Summer: fully development stage where the mineral consumption is at its peak. Beginning of summer and mid-summer ideally.
  4. None or Once in Autumn: need little as the mineral providing in this period might end up unused during the next season

Do You Need To Fertilize a Healthy Herb?

A good-looking herb, in general, does not need any fertilizer. However, good-looking does not imply that your herb is developing at its best, and it will stay like this forever.

Moreover, avoid waiting until seeing problems. Indeed, as discussed before, the mineral content in your potting soil is going to run out at some point. Hence, as mentioned previously, if your herb is already a year in the same pot without fertilizer, it is a good idea to start fertilizing (a mild 4-4-4, for instance). This applies even if it looks OK.

Over Fertilizing: Avoid These Three Mistakes

As strange as it might sound, the excess of fertilizer is one of the most common mistakes among beginner and enthusiast gardeners. Indeed, this happens because missing one of the steps I previously mentioned:

  1. Start fertilizing too early: countless times, you bought a nice herb, and you start fertilizing straight away. The potting soil is already mineral-rich. So providing extra is not going to do any good (on the opposite, you are damaging your herb). You need to wait a few months;
  2. Fertilize seedlings: seeds have all the nutrients packed with them. Fertilizing will probably kill them due to the high mineral content you will introduce.
  3. Following the fertilizer label religiously: one of the fertilizers I recently bought suggests to apply fertilizer once a week! This is an insanely high frequency, especially during the winter season. Do that, and your herb will have a way shorter lifespan. Try to resist the pulse and stick with the 4 times a year.
  4. Using a too strong fertilizer: the strength of fertilizer is given by the concentration of its nutrients (the higher the numbers N-P-K the stronger is the fertilizer). In the market you can find extremely strong fertilizer (the numbers are really high, above 30) that are also usually also quite unbalanced (one of the numbers is higher than the rest). An example is blood meal (high concentration of nitrogen) or pure potassium or phosphorous.

    This can be ideal in a specific situation in which you are totally sure that one of the nutrients is totally missing. However, this is hardly ever the case, and you might end up just damaging your herb. It is way easier to transplant your herb in a fresh potting soil;
  5. Fertilize as a way to revive dying herb: Yellow basil leaves, drooping chives, and many more signals do not imply that you herb need fertilizer. Reading around and chatting with expert gardeners, I realized that a bad-looking herb more often than not affected by problems totally unrelated to the lack of minerals in the soil.

    Thinks about root rot due to excessive watering, hidden aphids or spider mites, lack of light. These are, by far, the most common problems affecting your herbs. In this case, adding fertilizer will not have any benefits, and it might actually make the situation worse.

    Tip: Hence, if your herb is unwell, check for other signs that might explain those more common issues. Answer the following questions:
  1. Is your beneath the surface soil too wet?
  2. Is the herb receiving enough light?
  3. Is the ambient temperature to low for that specific herb (think about basil in winter in the northern hemisphere)?

Just addressing these questions, you are very likely to fix your herb without fertilizer.

Overfertilization Herbs: What is it? Why is it a Problem?

As mentioned earlier, overfertilization is way more common than under fertilization. Indeed, pushed by enthusiasm, especially beginners, are more prone to exaggerate.

What is the problem in providing a higher than the suggested amount of nutrients? After all, the more mineral, the better growth you might think, right? Unfortunately, this is not the case.

Indeed, the biggest problem caused by overfertilization is the excessive amount of minerals (those are the nutrients N-P-K) that build up in the soil over time. This is because the minerals that your herb will not absorb will remain in the potting soil in the form of salt. Different from open space, where minerals are recycled way more efficiently and quicker, this is not the case in a closed potted space. There the level of available minerals can raise way quicker.

Salt is a very bad companion for herbs because it “competes” with them for water. A high level of salt interferes with the proper development of the herb because it holds/absorbs water from the soil leaving the herb without water even if the soil is saturated. In the worst case in which the problem is not spotted in time, and the salt keeps rising, the salt will extract the water from the roots plant itself. This is the extreme cause of herb burnt (and that is what you might hear when some more expert gardeners refer to “fertilizer can burn your herbs”).

A second problem, equally important, is the “nutrients intoxication.” The herbs will try absorbing as many nutrients as they can. If such minerals are available in high concentration, your herb will suffer a variety of symptoms depending on how much and which nutrient(s) has been absorbed.

Do you want to know more about overfertilization? Click on the image below for a detailed guide on the most common sign of overfertilized herbs.

Related Questions

How do you fertilize herbs? This depends on the type of fertilizer adopted. Among the most common are those in solid or liquid form that must be added to a (relatively large) amount of water, creating a solution that is then added to the soil. Others (pellets) need to be placed within the soil; others need to be stuck into it (spikes).

Does mint need fertilizer? Mint as well, needs to be fertilized, if the soil is depleted of the 3 important minerals the other micronutrients. Inexpensive and easy-to-use soil testers are available both in online retailers and gardening shops.

Does basil need fertilizer? Yes, basil does need an N-P-K balanced fertilizer at the beginning of spring to promote foliage growth during summer. This is particularly true if the basil is indoor.

Further Readings

21 Easy Tips To Grow Massive Basil Indoor

2 Aspects For The Best Potting Soils and DIY Recipe

Why Earthworms Are Not Good For Your Indoor Herbs?

Is Coffee Ground Good For Your Herbs? 5 Proven Facts

The Fertilizer You Should Use For Your Indoor Basil

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