How to Fertilize Plants

What’s the difference between a good garden and a great one? It’s all in the way you feed your plants, among other things of course. But when your plants are at the mercy of your love and attention, knowing how to fertilize them correctly will make them adore you more.

Use a slow-release or granular fertilizer on plants for constant nutrient supply throughout the growing season. Alternatively, use a diluted liquid fertilizer as foliar sprays to give plants a quick nutrient boost. But remember, each plant has specific fertilizer needs to develop well.

Below, I elaborate on the when, the how, the frequency, and the amount of fertilizers you need to apply to indoor and outdoor plants:

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When Should I Fertilize My Plants?

Time of Day

When it comes to fertilizing plants, the timing is as crucial as the type and amount of fertilizer you use. Which is why the best time to fertilize your plants is early morning and late afternoon/early evening, when the temperature is much cooler. 

There are several reasons for this:

• Less Stress: At the peak of noon, plants are already stressed due to the high temperatures. Adding fertilizer can increase this stress, potentially harming the plants.
• Reduced Evaporation & Efficient Nutrient Intake: Because it’s cooler in the morning and late afternoon/early evening, plants can absorb water and nutrients easily without it evaporating quickly due to the heat.
• Leaf Scorch Prevention: Some liquid fertilizers that are used as foliar sprays can cause foliage burn if they remain on the leaves under the intense midday sun. This is because the water evaporates quickly, leaving behind the fertilizer salts on the leaves. By fertilizing in the morning or evening, you give the plants a chance to absorb the fertilizer before the sun is at its strongest.

Ok, but what about when the light has gone down completely, can you fertilize plants at night? Nightly fertilizing applications on plants are acceptable but not recommended. As strange as it sounds, even plants need sleep. Giving them the equivalent of energy drinks down their throat late at night is as bad as doing it as a human.

Similarly, the same can be said about watering your plants. If you want to know more, you can read all about it in this Worst Time to Water Plants article!

Frequency & Quantity | How to NOT kill your plants

Now that you know the best time to fertilize your plants, let’s talk about how often and how much you should be applying them. This one’s slightly tricky because each plant has specific fertilizing needs to grow their best. After all, over-fertilizing can harm your plants and under-fertilizing will slow its development, so you have to find the right balance. 

Here’s a general guideline on the frequency and quantity for different types of plants:

Important note: This list may not be applicable to all plants. Always check your specific plant’s needs and read through the product label to properly fertilize them. If you’re growing plants in a garden, it’s worth doing a soil test before amending the soil as well. 

If you noticed in the table above, houseplants require more frequent fertilization than the rest. This is simply because potted plants generally require more frequent fertilization than those in the ground because they have limited soil volume. Cacti & succulents are a bit of an exception to this rule because they’re used to thriving with less nutrients in their surroundings. 

If you’re still unsure, here are a few quick tips to safely fertilize your plants without killing them: 

• Only use or apply fertilizer on plants in spring and summer, at least once a month.
• Always follow the recommended dosage on the fertilizer label. More isn’t always better.
• Water the soil before and after applying fertilizers to prevent root burn.
• Avoid fertilizing when the soil is extremely dry or overly wet. This is also why you shouldn’t fertilize plants after it rains because it may just get washed away into nearby watering systems rather than into the soil. 

Seasons

Spring

This season is when plants break out of their dormancy and begin a new cycle of growth. Which is why proper fertilization during this period can set the stage for a healthy and productive season. Obviously, this doesn’t account for tropical, equatorial, and polar regions where they don’t experience 4 seasons.

In the Northern parts of the world, Spring is from March to May. This means you can start fertilizing as early as March. But for those living down South, it’ll be from September to November. Just ensure that the soil is workable and not too wet when applying fertilizers.

Summer

During this season, the plants are in their active growth phase. You know the one – that sweltering month filled with beach days and awkward teenager phases. Fertilizing plants in summer ensures healthy growth, vibrant blooms, and a bountiful harvest. It usually starts from June to August in Northern areas. But for Southern regions, it begins from December through February. I know, it’s a bit odd, but hey, I don’t control the sun. 

However, the summer season also brings with it challenges like high temperatures and intense sunlight, which can affect the way plants utilize nutrients. During extremely hot weather in summer, it’s best to avoid fertilizing plants, even more so if you notice them wilting. Stick to the cooler morning times and be sure to water your plants more frequently to prevent them from drying out. It may also be beneficial to add some shade for your plants.

Autumn/Fall

This transitional season bridges the gap between the warm summer months and the cold winter. As the days become shorter and temperatures drop, plants undergo various changes to prepare for the upcoming winter. If you live in the Northern hemisphere, Autumn typically goes from September to November. But if you are in the Southern hemisphere, it starts from March and ends in May. 

For the most part, fertilizing plants in the Fall is beneficial for certain plants, especially those that are preparing for dormancy. It helps plants store essential nutrients that they will use during the winter months. This stored energy will then be utilized in the spring when the plants begin to grow again. However, it’s essential to choose the right type of fertilizer and apply it at the correct time to avoid any potential damage to the plants.

Winter

This cool period of time is when many plants go dormant, conserving energy and preparing for the growth spurt of spring. Which is why it’s common practice to avoid fertilizing plants in Winter. In the Northern regions, it’s normal to expect winter from December to February. But for those in the Southern areas, it’s typically from June to August. 

In some special cases, like indoor plants that are still actively growing, fertilizing them once a month may be helpful for their development. But for most plants, it’s better to err on the side of caution and stop fertilizing altogether. Otherwise, you’re more likely to over-fertilize and cause nutrient burn or even damage the plant’s roots. 

How to Fertilize Indoor Plants

Indoor plants are pretty straightforward to fertilize. But before we get into that, let’s talk about the various types of fertilizers and how they’re usually applied on plants.

Types of Fertilizers & Its Application

1. Liquid Fertilizer.

This fertilizer type is usually very concentrated and is typically diluted in water beforehand. It’s standardly used in hydroponics or any plants that grow in water. Like most fertilizers, the recommended mixture levels will vary based on a plant’s needs and each product label, so please follow those carefully. You can use a watering can or even a bowl before pouring the diluted liquid fertilizer around the plant’s soil. If it’s a foliar fertilizer, aim it at the leaves and use it sparingly during the growing season. 

2. Granular Fertilizer.

This particular fertilizer comes in the form of solid particles. It’s the most common fertilizer form used, both in indoor and outdoor plants. To apply, measure the required amount of granular fertilizer based on the product’s instructions and the size of your plant. Sprinkle the granules evenly around the base of the plant, avoiding direct contact with the stems or leaves. After application, water the plant thoroughly to help dissolve the granules.

How to Fertilize Potted Plants

Potted plants, unlike those in the ground, have limited access to natural nutrients. Over time, the soil in pots can become depleted of essential nutrients. This is why regular fertilization is necessary to ensure the plant’s best growth. Here are a few tips to help you out:

• Always read the label to ensure the fertilizer is suitable for your specific plant type. When in doubt, use an all-purpose fertilizer. 
• Water the plant thoroughly a day before fertilizing. This ensures the soil is moist and ready to absorb the nutrients.
• Follow the recommended dosage on the fertilizer package. Over-fertilizing can harm the plant.
• Apply the fertilizer evenly around the base of the plant, avoiding direct contact with the stem. If you’re using foliar sprays, apply it directly on the plant’s leaves.
• Flush out leftover fertilizer salts in the plant’s soil by placing it under running water on a monthly basis. You can also do this when you see signs of white residue around the pot’s rims.
• Don’t fertilize repotted or recently transplanted plants. Wait a week or two to allow the plant to settle in its new environment before fertilizing.
• Keep an eye on your plants after fertilizing. Yellowing leaves, stunted growth, or burnt tips can be signs of over-fertilization.
• Adjust your fertilization routine based on the plant’s response. If it appears stressed, consider reducing the fertilizer amount or frequency.

How to Fertilize Plants in LECA

LECA stands for Lightweight Expanded Clay Aggregate. It’s a soilless medium that has gained popularity, especially for indoor plants. It’s somewhat like a semi-hydroponic system, except the plant is not being fully submerged in water. Fun fact: LECA is also used in hydroponics systems!

LECA is said to be mess-free when compared to soil and has a decent water-holding capacity. Its ball-like shape also creates adequate gaps in between layers, providing good air circulation for the plant’s roots. But when it comes to fertilizing plants in LECA, there’s a specific way of doing it. Here’s how to get started:

• Use a water-soluble fertilizer. This ensures that the nutrients will be readily taken up by the plant’s roots.
• Follow the instructions on the fertilizer’s label. It will provide guidance on how much to use to make sure you use the correct dosage without overfertilizing your plant.
• Monitor the plant’s pH Levels. The pH of the water can affect how plants absorb nutrients. Aim for a pH level between 5.5 and 6.5 for most plants, but some will have specific levels to adhere to.
• Regularly refresh or change the water. Over time, the water in your LECA pot can accumulate salts from the fertilizer. To prevent this, regularly flush the system with clean water.
• Keep an eye on your plant. They will tell you if they’re getting too much or too little fertilizer. Some symptoms that suggest nutrient imbalances include yellow leaves and brown tips.

How to Fertilize Plants in Water

Plants that grow in water are often referred to as hydroponic or aquatic plants. They require a different fertilizing approach to regular soil-grown plants. The absence of soil means that plants rely solely on the nutrients provided in the water to grow and thrive. Here’s a concise guide on how to do it effectively:

• Use water-soluble fertilizer specifically designed for hydroponic or aquatic plants. These fertilizers are formulated to provide all the essential nutrients that plants need in a form that’s easily absorbed through water. For aquarium-based plants, there are also pressed tablets that can be pushed into the substrate or gravel, slowly releasing nutrients over time to where the plant’s roots are. If you’re interested in bringing home outdoor plants into your aquarium, you can read more about it in our River & Pond Plants in an Aquarium article!
• Follow the manufacturer’s recommendations to determine the correct dosage. Over-fertilizing can lead to nutrient burn, while under-fertilizing can result in nutrient deficiencies.
• Monitor the plant’s water quality like the pH, hardness, and nutrient levels. This will help you adjust the fertilizer dosage as needed and ensure that plants are getting the right amount of nutrients.
• Replace about 10-20% of the water in your aquatic setup every week. This helps in removing excess nutrients and prevents the buildup of harmful substances.
• Keep an eye on your plants and their growth. If there are signs of nutrient deficiencies, adjust your fertilization routine.
• Avoid overcrowding, ensuring that the plants have enough space to grow. Limited space will lead to plants competing for nutrients, leading to suboptimal growth for all plants in the setup.

How to Fertilize Outdoor Plants

Compared to indoor plants, outdoor plants are more independent in getting their nutrients from the ground. But that doesn’t mean they don’t appreciate a dash of fertilizer here and there. Here are tips on how to fertilize outdoor plants effectively:

• Do a soil test. You can do this through your local county extension office, who will determine the pH and nutrient levels of your soil. By knowing this, it’ll let you know of any deficiencies you need to amend before planting. However, this doesn’t apply to raised bed gardens as that requires you to use fresh potting soil above ground anyway.
• Apply organic fertilizer like compost and aged manure in the soil before planting. These are usually the best type of fertilizers for outdoor plants. It ensures that the ground is already rich with nutrients, which the plants can easily take up as they grow. Alternatively, you can side-dress the ground with synthetic fertilizers. But better yet, try growing cover crops like legumes and clovers and till them back into the ground as green manure. If you’ve also done the soil test, you can add your soil amendments during this time too. 
• Use foliar sprays on some plants to give them a quick boost. Even though there may be enough nutrients for the plants in the soil, additional fertilizer used in moderation may help increase yield. For example:
  • Strawberries: Needs a balanced NPK fertilizer when the plant starts producing leaves and again when the fruits start to form.
  • Beans: Requires high-Phosphorus fertilizer during planting and when the beans start to appear. 
  • Raspberries: Only needs balanced NPK fertilizer after the plant is established. But needs higher doses every year.
  • Pineapples: Use foliar spray like fish emulsion or kelp extract on the soil and leaves every 8-10 weeks.
  • Fast-growing herbs like basil, cilantro, parsley, and chives do best with a monthly foliar spray with high-Nitrogen fertilizers. But if you’re growing slow-growing herbs like rosemary, sage, and thyme, they usually don’t need additional fertilizers.

Frequently Asked Questions:

Should you water plants before or after fertilizing?

It’s best to water plants before fertilizing. This prepares the soil to absorb nutrients effectively and reduces the risk of root burn from concentrated fertilizers. After applying fertilizer, water lightly again to help distribute the nutrients.

How do you fertilize plants naturally?

Here’s how to fertilize plants naturally:

• Use compost or well-decomposed, aged manure to enrich the soil.
• Apply organic mulch like leaves or straw to retain moisture and add nutrients.
• Use natural liquid fertilizers like worm tea, compost tea, or fish emulsion. You can read more about this in our Compost tea article!
• Plant cover crops aka green manure like clovers and till them back to add nutrients into the soil.
• Rotate crops to maintain soil health.
• Encourage beneficial bugs and worms for natural soil aeration and fertilization. If you’re interested in this topic, have a read on our Beneficial Bugs article!

Can you fertilize plants too much?

Over-fertilizing plants is bad for their health and also for the environment. Excessive fertilization can lead to a buildup of salts in the soil, causing root burn and inhibiting water uptake. This can result in yellowing leaves, stunted growth, and even plant death. Other than that, leftover fertilizers can be washed away, leaching into open waterways and causing pollution.

How to fertilize plants without fertilizer?

To fertilize plants without commercial inorganic/synthetic fertilizer, use organic methods:

• Add decomposed organic matter such as compost and aged manure to soil.
• Use worm castings from vermicompost to enrich soil. You can read more about this in our Vermicomposting article!
• Apply organic mulch around the plant’s soil to retain moisture. Overtime, it’ll break down and add nutrients to the soil.
• Plant beneficial cover crops like clover and beans to add nutrients back into the soil by tilling them after they’ve grown.

Should you fertilize plants during a drought?

Do not fertilize plants during a drought. Without adequate water, fertilizers can be too concentrated and cause root burn, further stressing the plant. Instead, focus on water conservation and providing shade.

Should you fertilize plants when they are blooming?

It’s recommended to fertilize flowering plants during blooming with a high Phosphorus fertilizer. Don’t use all-purpose or high Nitrogen fertilizers as this will cause more leaves to grow, instead of blooms. This same method is also applicable to fruiting plants.

How does poop fertilize plants?

When poop or manure decomposes, important nutrients are released into the soil, enriching it and promoting plant growth. Additionally, organic matter in poop improves soil structure and water retention. But it’s important to let it age and decompose first because using fresh poop or manure contains pathogens and may burn the plant’s roots. 

How does vinegar fertilize plants?

Using vinegar is not recommended to fertilize plants. Some have said using it to lower the soil’s pH is useful but there’s not enough evidence to support those claims. There are also people who use it as a weed and bug killer, but it’s bound to cause damage to plants’ leaves in its concentrated form.

How do we use Ammonia to fertilize plants?

It’s not advisable to use pure, concentrated Ammonia (NH₃) to fertilize plants. When it is applied in soil, it converts to ammonium (NH₄⁺), which plants can absorb as a nitrogen source. You can dilute it with water before application for a quick fix, but I highly don’t recommend trying this method of fertilization.

Can plants fertilize themselves?

Many plants in the wild are able to fertilize themselves via decomposed matter from falling leaves and animal droppings, but not indoor or potted plants. Some outdoor plants even have specialized functions to help them thrive regardless of a deficiency in a soil. For example, legumes can convert Nitrogen in the air with the bacteria on their roots and enrich the soil with more Nitrogen. 

References:

1. Clemson Cooperative Extension. (n.d.). Fertilizing trees & shrubs. Clemson University. Retrieved August 23, 2023, from https://hgic.clemson.edu/factsheet/fertilizing-trees-shrubs/
2. Gardening Solutions. (n.d.). Fertilizer basics. University of Florida IFAS Extension. Retrieved August 23, 2023, from https://gardeningsolutions.ifas.ufl.edu/care/fertilizer/fertilizer.html
3. Home and Garden Information Center. (n.d.). Care of annuals and perennials. University of Maryland Extension. Retrieved August 23, 2023, from https://extension.umd.edu/resource/care-annuals-and-perennials
4. Home and Garden Information Center. (n.d.). Fertilizing vegetables in the home garden. University of Maryland Extension. Retrieved August 23, 2023, from https://extension.umd.edu/resource/fertilizing-vegetables
5. Leca Balls. (n.d.). University of Washington Botanic Gardens. Retrieved August 23, 2023, from https://depts.washington.edu/hortlib/pal/leca-balls/
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7. Owen, J. S., & Mize, C. (n.d.). Quick guide to fertilizing plants. University of Minnesota Extension. Retrieved August 23, 2023, from https://extension.umn.edu/manage-soil-nutrients/quick-guide-fertilizing-plants
8. Owen, J. S., & Mize, C. (n.d.). Small-scale hydroponics. University of Minnesota Extension. Retrieved August 23, 2023, from https://extension.umn.edu/how/small-scale-hydroponics
9. Owen, J. S., & Mize, C. (n.d.). Fertilizing houseplants. University of Connecticut Soil Nutrient Analysis Laboratory. Retrieved August 23, 2023, from https://soiltesting.cahnr.uconn.edu/fertilizing-houseplants/
10. Ruter, J. (n.d.). Fertilizing ornamental plants in the landscape. University of Georgia Extension. Retrieved August 23, 2023, from https://secure.caes.uga.edu/extension/publications/files/pdf/C%201179_1.PDF
11. Sippel, K., & McNeilan, R. (n.d.). Know what your plants need before fertilizing. Oregon State University Extension Service. Retrieved August 23, 2023, from https://today.oregonstate.edu/news/know-what-your-plants-need-fertilizing
12. Zhang, Y., & Muehlbauer, M. (2023). Growth and physiological responses of container-grown Norway spruce seedlings to controlled-release fertilizer type and rate. New Forests, 54(2), 217-235. https://link.springer.com/article/10.1007/s11056-023-09973-x