All About Fertilizing Houseplants


Note: I republished this blog with permission from Folia Design, a Quebec City houseplant boutique. The author is Réal Dumoulin. I did the translation from French into English.

I must admit I was impressed with the article from the start, but all the more so when I realized the author had quoted me! (How often does that happen!) So, just maybe that influenced my desire to republish this … but I like to think I did so simply because it’s a really great article that covers a complicated subject quite succinctly.

Enjoy! .jpg

It doesn’t take much fertilizer to make houseplants happy! Source:

There are many myths circulating about the importance of fertilizing plants, especially houseplants. Most people think that fertilization plays a major role in the health of their plants when in fact it’s only a minor factor in their development. What matters most in the life of a houseplant is the quality of the soil, lighting, watering, humidity and ambient temperature. However, that doesn’t mean you can completely neglect fertilization either: it still plays a small role in the quality and beauty of our indoor plants.

When Should I Fertilize?

The main rule is to only fertilize a plant when it is actively growing!

  • In general, you don’t need to fertilize plants during the winter in the average home. Most will be semi-dormant and fertilizing then would only stimulate weak and etiolated growth, susceptible to insects and diseases. However, starting at the end of February, the increasing luminosity leads to a growth spurt in most indoor plants. That’s a sign it’s time to fertilize.
  • However, there are exceptions. Any plant that is clearly growing in the winter can be fertilized, although normally at a lower rate than the one recommended on the fertilizer label, say at about ¼ to ½ of the usual rate. Plants that are getting especially bright light, for example under fluorescent lights, in a solarium or in front of a south-facing window, may continue to grow and even bloom during the winter and they’re the ones that will likely need fertilization. In addition, some plants, such as cyclamens, houseplant azaleas, gardenias, etc., are naturally in full bloom during the winter, and therefore need regular fertilizing at that season.
  • One common error is fertilizing a plant when it’s clearly in distress, something you should always avoid! When a plant is in bad shape, you need to analyze the situation and treat it according to its needs. Is it receiving insufficient light? Too much or not enough water? Is its soil too compact? Is it contaminated with mineral salts (unabsorbed fertilizer from past treatments)? Fertilizing a plant in distress may out-and-out kill it!
  • Generally, you won’t need to fertilize after repotting or top dressing, because the potting mix used already contained fertilizer. Wait at least 2 to 4 weeks before starting to fertilize a plant after either treatment.
  • Plants won’t usually need fertilizing for about 6 weeks after purchase, as they were likely treated to a good fertilizing regime in the store.

The normal period for fertilizing houseplants is from the end of February to the month of September whether they spend the summer indoors or out.

Which Fertilizer to Choose?


It really doesn’t matter which fertilizer you use on your houseplants. Even lawn fertilizer will do! Source:

Stores offer a huge number of different fertilizers in stores and they come in different forms and formats, plus they can be chemical or organic. Naturally, you’ll want to choose the best for your plants.

However, it’s important to understand that most fertilizers are about equally good and you can use almost any one on any plant. Garden writer Larry Hodgson rightly points out that plants can’t read fertilizer labels. In other words, they’ll do fine no matter which fertilizer you use.

Slow-Release or Quick-Release Fertilizers?

Some fertilizers are in the form of granules or spikes and are slow release, that is to say that they’ll fertilize the plant over a long period. Often, a single spring application is sufficient, although a second one in early July may be useful.

Most fertilizers, though, are quick release and come in either liquid or powder form. They’re designed to be diluted in water before use, according to the manufacturer’s instructions. Just water the plant’s potting mix with diluted fertilizer, although it’s best not to fertilize plants when their soil is dry, as this can harm the plant’s roots and, eventually, the plant itself.

Foliar fertilizer is yet another kind of fertilizer. It’s designed to spray directly on the foliage and yes, plants do absorb fertilizer through their leaves. Often, liquid seaweed, properly diluted, is used as foliar fertilizer.

20-20-20? / 10-30-10? / 15-30-15? / 30-10-10?

Oh my! All those numbers! What’s a houseplant enthusiast to think? But remember from above that plants can’t read fertilizer labels and really don’t care what you use! In general, a balanced fertilized like 20-20-20 or something similar is ideal for all plants. Liquid seaweed fertilizers are particularly interesting for both foliar use and for watering into potting soil.

What About Compost?

Compost is not really a fertilizer, but a product that improves the soil’s structure and texture so that it better retains water and fertilizers. By all means, do add some to your potted plants, but you’ll still need to fertilize at some point.

Should I Use Organic or Chemical Fertilizer?


Chemical fertilizers often come in bright artificial colors. Source:

People have a wide range of opinions on that subject, but in fact, plants really don’t care which type you use. The choice will likely be based more on your personal values than on the plant’s needs. You can easily find organic fertilizers such as liquid seaweed if that’s what your choice.

Avoid Over-Fertilizing!

Many people use too much fertilizer, thinking that the more they give, the more beautiful and healthier their plants will be … and that’s a serious mistake! Far more plants die of over-fertilization than a lack of minerals. Excess minerals lead to tall, unhealthy growth and leaves plants susceptible to diseases and insects. In addition, the excess leads to mineral imbalances, harming the plant even further.

In short, a balanced fertilization you apply according to the plants’ needs will help your houseplants grow and flourish … but any excess will be

Analyze This!


20150411AIf you’re starting to take gardening seriously, you really should have an occasional soil test done. It’s important to understand the soil you garden in, especially just when you’re starting a new flower bed or vegetable garden.

The test will tell you, among other things, whether the soil’s pH is too high or too low (that is, whether it is too alkaline or too acid) and if any vital minerals are missing. By correcting your soil’s flaws before you even begin to start planting, you’ll get much better results. And even if you decide to learn to live with the soil conditions Mother Nature gave you (or, more likely, with the poor quality subsoil left you by the contractor who removed your property’s topsoil during construction), at least you’ll know where you stand and you can then choose plants that will do well under those conditions.

What Do Soil Tests Analyze?

In general, a soil test will include the following elements:

20150411E1. Soil type/particle size analysis: is it clay, sand or loam? Does it hold water and minerals or allow them to drain away? Is there enough organic matter?

2. Acidity/alkalinity: it will measure the soil’s pH to determine its acidity or alkalinity. The pH scale for garden soils ranges from about 4 (very acidic) to a bit above 8 (very alkaline), with 7 being neutral. In general, plants require a slightly acidic to neutral pH, from 6.0 to 6.5, to do well, but acid-loving plants (rhododendrons, blueberries, heathers, etc.) prefer soil that is distinctly  acid (a ph of 4.5 to 6.0). The test also looks at the buffer pH, a factor that will help determine the amount of lime you’ll need to add to correct soil’s acidity and achieve the desired pH.

2. Phosphorus (P) Content: it will measure the amount of phosphoric acid present in the soil. Phosphorus is needed to promote root development and overall plant growth. It also plays an important role in the production and ripening of fruit.

3. Potassium (K) Content: the test will determine the available potassium in the soil. Potassium makes plants more resistant to insects, disease and stress such as drought. It plays an important role in flowering, improving fruit flavor and intensifying their colors.

4. Micronutrients: these are minerals that are essential to the plant’s health, but needed in very small quantities, such as calcium, magnesium and iron. Calcium favours the growth of young roots and promotes ripening. Magnesium plays an important role in the green coloration of the foliage, ensures proper ripening of fruit and promotes the plant’s absorption of phosphorus and nitrogen. Iron promotes the development of darker green leaves and is an essential element for plant growth. Don’t underestimate the importance of micronutrients, even if the plant only requires trace levels of these minerals: their absence will cause a deficiency and lead to poor growth and reduced yields.

Not Included in the Soil Test

It may seem odd that the laboratory soil tests don’t check the soil’s nitrogen (N) level even though it is the most important mineral plants need to grow well. But nitrogen is also the most volatile mineral and its content rises and falls depending on many factors, including temperature, moisture level, and microbial activity. Soil nitrogen test results are therefore only valuable for a short time and even then only under the conditions under which the test was made. For that reason, the results of any test will always recommend the addition of some quantity of nitrogen, but by using other criteria to estimate the situation in your garden.

Where to Go for a Soil Test

Most garden centers and even many hardware stores offer a soil test service… but not in a very visible way. You won’t find a small laboratory in the center of the store in full view of everyone. Instead, you have to ask an employee about the service. In general, you will be given a bag or container in which to put your sample and a form to be completed, including what you intend to grow in the garden whose soil you want analyzed.

Then simply take the kit home, take soil samples as recommended below, and bring them back to the store.

When to Do a Soil Test?

There is no specific season for having soil tested. You can do it at any time of the year… at least, when the ground is not frozen or sopping wet. Most gardeners have theirs done in the spring, just before the growing season starts. Autumn however is perhaps the best time: if you need to apply lime, which is a very slow-acting product, a fall application will give it time to work all winter so the soil will be showing results by spring.

If you have recently limed your yard or garden, it would be better to wait 5 or 6 months before doing the test. And wait a few weeks after applying fertilizer or compost as well. By then, the soil will have regained its balance and therefore the test will more accurate results.

Taking a Sample

Each test should be of one garden only, whether a vegetable garden, a lawn, or a flowerbed, as the recommended treatments will vary depending on the plants grown. If you have a special garden with soil needs outside of the normal range, for example a rhododendron garden, a separate test is also required. Serious gardeners will often do one test a year, each time in a different garden, starting with the most urgent (a lawn in poor shape, for example) to spread the cost of testing and treatments over time.

20150410BBefore you begin, thoroughly wash your sampling tool (professionals use a core drill, but a garden trowel is the home gardener’s usual choice), rinsing well to remove all traces of soap. Take the equivalent of a few large tablespoonfuls about 6 to 12 inches (15 to 30 cm) deep, in the root zone. Do not use surface soil: it often contains elements that can skew the results. Pour each sample into the test bag or container, then take other samples at different locations of the same garden (vegetable garden, lawn, flowerbed, etc.) for a total of about six samples (more if the garden is very large): this will ensure a more comprehensive picture of the situation that a single sample.

If necessary, remove any stones, sticks, etc. from the sample, wearing gloves (do not touch the sample with your hands, as they can contaminate the soil and affect the results), then blend well. Each test requires about 1 cup (250 ml) of soil; if you have too much, remove the surplus.

200452424-001It’s also very important to complete the form that accompanies  the kit. First, to identify yourself so that the results get back to you, but also so the lab knows what you want to do with the garden. That’s because there will be different recommendations for a lawn, a flowerbed, a vegetable garden or even a space that will be dedicated to a specific group of plants.

Now return the sample to the merchant. Normally, he will contact you in about two weeks with the results. The cost? The going rate in 2015 seems to be about $20.

Test Results

20150411EANGLAISExpect very detailed results with specific recommendations, like “apply product X at such a dose and such a frequency”. In high-rainfall areas, soils tend to be acidic, so there may be lime to apply; in low-rainfall areas, soils tend to be alkaline, so sulphur will likely be recommended. In either case, there may well be fertilizers and organic amendments such as compost to apply. Don’t be surprised to discover that the recommended treatment for your lawn is not the same as for the flowerbed or vegetable garden, because each of these environments has different soil needs.


Every garden really should be tested every 4 or 5 years. That way if there are any changes occurring (for example, the soil is starting to return to its original pH), you’ll be aware of them before the plants begin to suffer. You’ll also be forewarned if your soil’s mineral supply is becoming exhausted.

For a vegetable garden, test the soil even more often, every 2 or 3 years, because you won’t want to skimp on the quality of its soil, not if you want vegetables of the highest quality!

The average home gardener can follow the above rule for his lawn (a test every 4 to 5 years), but then, most gardeners are content with a lawn that is green and fairly weed-free. If you want a perfect golf green type lawn, though, biannual or annual testing may be required. When you’re a lawn nut, nothing is too good for your personal green space!