Can I use orchid fertilizer on houseplants?

There is a lot of information floating around about fertilizing orchids, but much of it is wrong. For example, the whole concept of “orchid fertilizer” is a myth because there is no such thing.

It’s time that orchid growers understand fertilizer better. It will benefit your plants and save you money.

Orchid Fertilizer Myths – Grow More Flowers

There is No Such Thing as Orchid Fertilizer!

That statement is not entirely correct. Some consumers and the marketing department of many fertilizer companies believe in orchid fertilizer, but it does not really exist.

Let’s compare two products; Dyna-gro Orhicd-pro 7-8-6 fertilizer and Dyna-gro Foliage Pro 9-3-6 general plant food. The ingredient list for both products is identical and includes ammonium nitrate, potassium nitrate, potassium phosphate, ammonium phosphate, calcium nitrate, cobalt sulfate, magnesium sulfate, and some minor nutrients. You may recognize this list of chemicals as the main ingredients in just about every fertilizer including ones for lawns, houseplants, flower beds, roses and even tomatoes. There is no magic ingredient that is used specifically by orchids.

Still not convinced? You might be familiar with the special Michigan State University’s (MSU) Magic Orchid Fertilizer. It is all the rage and has shown some impressive results. It was developed as an all-purpose general fertilizer and is used on Easter lilies, poinsettias, annual bedding flowers, perennials, ferns, conifers, cacti, succulents and hundreds of tropical plants. It was never designed for orchids.

The ingredients in all so-called orchid fertilizer are exactly the same as the ones used for every other type of plant. The amounts of each ingredient may vary, but the actual ingredients are the same. That means you can use any fertilizer on your orchids because there is no such thing as special orchid fertilizer.

Which Orchid Fertilizer is the Best?

Every orchid grower wants to know which orchid fertilizer is the best. Let’s see what the experts say.

Here is a fun exercise you can do. Google for images of orchid fertilizer and see what you find. To help you out I did this and made a collage of some images.

Can you spot the best orchid fertilizer?

These are popular orchid fertilizers supplied by experts. One of them has a NPK of 7-8-6 with the nutrients in about equal amounts. But then there is a 13-2-13 which suggests orchids don’t need a lot of phosphorus. A 20-10-20 suggests orchids need equal amounts of nitrogen and potassium, but the 30-10-10 fertilizer suggests orchids need 3 times more nitrogen than potassium. The so-called “experts” don’t seem to agree on what is best.

Which of these fertilizers is the best? None.

There are several reasons why none of these is the best. The nutrients you need, depends on various other factors:

• nutrients supplied by the media – bark adds more nutrients than clay pellets
• nutrients in the water you use – tap water can contains lots of calcium and magnesium, and some potassium; rain water or RO water contains very little of anything
• how you fertilize and water
• type of orchid

The reality is that for some nutrients, plants are able to only absorb what they need and the excess just gets washed down the drain. In general, plants use the main nutrients in a ratio of 3-1-2 and this goes for orchids as well. There is some evidence that a bit more potassium, such as a 3-1-3 ratio, might be better for orchids. None of the above fertilizers have this ratio.

What Does NPK Mean?

Gardeners use the term NPK frequently, and it is listed on fertilizer packages, but many people do not understand what it means. I had no problem finding reliable sources of orchid information that say something like, “the numbers refer to the percentage by weight of nitrogen, phosphorus, and potassium”, which is wrong.

The N value is the % nitrogen. The P and K values are the % P2O5 and % K2O and NOT the % P and % K.

For a more detailed discussion on this see, Fertilizer NPK Ratios – What Do They Really Mean?

Phalaenopsis showing difference between a flower spike and a root

Fertilizer Should be Applied Weakly Weekly

This is common advice and as a general statement it does make some sense. Orchids can be killed by high salt levels, and all of the nutrients in fertilizer are salts. Applying small amounts, especially by new growers who are more familiar with fertilizing other types of house plants, is good advice. But is it the best advice for growing really healthy plants that flower a lot?

This advice is usually coupled with a suggestion to only use a half or even a quarter of the strength recommended by the manufacturer. This might make sense if you are using house plant fertilizer since house plants can take more fertilizer, so orchids should get less. But does this advice also apply to orchid fertilizer? Should you use 1/4 of the recommendation found on orchid fertilizer packages? Surely the instructions on orchid fertilizer are correct for orchids? Why would you cut the recommended amount in half or quarters?

I decided to look at this a bit closer, with a focus on the amount of nitrogen being given to plants (i.e. numbers are normalized for nitrogen). Both the Miracle-gro water soluble plant food 20-20-20 and Jack’s All Purpose 20-20-20 recommends 132 ppm N for indoor plants. If the right amount for orchids is half or quarter of this, one would expect orchid fertilizers to recommend around 50 ppm. Here is what some orchid fertilizer recommend.

• Miracle-gro Orchid 30-10-10: 100 ppm
• Grow More Orchid 20-20-20: 264 ppm
• Jack’s Classic Orchid 30-10-10: 200 ppm
• Plant Products Orchid 25-10-10: 330 ppm for phals, cymbidium and vanda, and 165 ppm for cattleya, dendrobium, paphiopedilum and miltonia

There are two things to note. First of all, the numbers are very inconsistent. How can they all be right?

Secondly, on average, the numbers for orchids are higher than the recommendations for house plants.

Based on these numbers, the recommendation to use half or one quarter of what is on the label does not make sense, either for orchid fertilizer or general purpose fertilizer.

Using the Right Amount of Nitrogen

Effect of fertilizer levels on orchid growth, photo Yin-Tung Wang

It seems clear that the labels on fertilizer containers can’t be trusted. So what is the right amount of fertilizer?

If any nutrient is deficient, plants will have trouble growing and flowering, but it turns out that nitrogen is the one nutrient that has the largest effect on plant growth and flowering, provided that other nutrients are available in reasonable amounts. Higher nitrogen levels result in more leaf growth, more flower stem branching and more flowers.

Various studies have looked at nitrogen levels to determine the best one for orchids.

Phalaenopsis is the most popular type of orchid and it has been studied the most. It turns out that a nitrogen level of 100 ppm will produce good plants that flower regularly but increasing that level to 200 ppm will produce larger plants and more flowers. Levels above 200 ppm can be detrimental to phals. Hobbyists are better keeping levels between 100 and 150 ppm.

There is some evidence that Cattleya and other slower growing plants do better with lower amounts of nitrogen, but there is also evidence that higher amounts work just as well. If you are growing a wide range of species it might be safest to lean towards the 100 ppm range.

Calculating 100 ppm

Scientists measure nutrients in ppm and gardeners measure in teaspoons per gal. This section will help you convert one set of units to the other.

A teaspoon of soluble fertilizer weighs about 5 g (will vary depending on the product).

One tsp of a 10% nitrogen fertilizer added to one gallon of water will produce a 132 ppm nitrogen solution. The same solution will be produced by using 1/2 tsp of a 20% fertilizer or 1/3 tsp of a 30%.

Does Urea Harm Orchids?

The most critical nutrient affecting growth is nitrogen and it can be supplied in three forms; nitrate (NO3), ammonium (NH4) and urea (CO(NH₂)₂). Nitrate and ammonium are ions in water that can be absorbed directly by orchid roots. Many believe that the organic molecule, urea, can’t be absorbed by orchids. while others claim that “urea can take up to a year for your orchid to break
it down.” Therefore you should not use urea.

When urea mixes with water it can be easily decomposed by bacteria into ammonium. The media used in most pots contains enough bacteria for this to take place fairly quickly (in hours or days) and in the ammonium form it is easily absorbed by roots. When discussing forms of nitrogen in fertilizer, urea and ammonium are essentially the same thing.

What is even more surprising is that orchid roots, unlike the roots of many plants, can actually absorb urea directly. It is believed that orchids have evolved to make better use of partially decomposed organic matter and therefore have developed this capability.

There is no problem using a fertilizer containing urea but as you will see below it is best not to use one where all of the nitrogen is in the form of urea or ammonium.

Which Form of Nitrogen is Best?

Orchids can use nitrate, ammonium and urea, and commercial fertilizer contains various amounts of these. Do your plants care?

It turns out they do care and the various forms are not exactly the same. The way in which roots absorb these nutrients is different but more important is their effect on pH. As plants use urea and ammonium they make the media more acidic. Nitrate makes it more alkaline. If your water source is alkaline the use of acidic forms of nitrogen can be an advantage.

Ammonium requires less energy for the plant to use it, which is a benefit, but it can also become toxic and inhibit the uptake of cations like potassium, calcium and magnesium.

Growth of phals in different ratios of nitrate and ammonium have shown that they grow larger leaves, spike earlier, produce more flowers and larger flowers with higher levels of nitrate.

Effect of nitrate and ammonium on phals, photo by Yin-Tung Wang

A similar study using phalaenopsis and dendrobium shows that the best growth and accumulation of nutrients takes place with at least 25% ammonium.

Phalaenopsis and dendrobium do not grow and flower well with 100% ammonium or urea, particularly when planted in sphagnum moss. They grow best with 60 – 75% nitrate and the balance either ammonium or urea. This is probably also good advice for other types of orchids.

Do You Need an Orchid Bloom Booster?

Orchid bloom booster fertilizer usually has a relatively high level of phosphorus. The advice for using a bloom booster is inconsistent and here is what some so-called experts, including fertilizer manufacturers say.

• Use it all year long
• Use it a couple of times a year
• Use it when you want plants to flower
• Use it after a phal shows the flower spike

This inconsistency should be a red flag and should get you to ask for some scientific data to support its use. What does science say? Studies using phals has shown that higher levels of phosphate actually decrease flowering. ” In one study, phosphorus varying from 22 to 242 ppm (with 100 or 200 ppm of nitrogen) did not influence growth or flowering of phalaenopsis. Preliminary research indicates that 25 to 50 ppm phosphorus is adequate to grow an excellent crop”.

Bloom boosters are “marketing hype” and do not really exist. This is not only true for orchids, but also for other house plants and even garden plants. High phosphorus does not produce more flowers.

Since high levels of phosphorus can reduce flowering it becomes clear that using a balanced fertilizer, like 10-10-10, is also not a good idea. Nitrogen should be higher than the phosphorus by a factor of 4.

To find out more about blooming orchids see: How To Bloom Orchids.

Should You Water First and Then Fertilize?

Orchid root cross section showing velamen, photo by Phil Gates (Beyond the Human Eye)

I have seen this recommendation consistently over the past 30 years and it has never made any sense to me. The idea here is that you should water your orchids first, to wet the media and the roots. Then fertilize. This is usually coupled with some talk about keeping salt levels low and preventing salt accumulation.

The American Orchid Society suggests “some mixes, especially those containing pine bark, can be difficult to wet through, and so should be pre-watered with plain water and left to sit a few minutes until the medium is completely damp before fertilizing. This helps reduce salt buildup and the possibility of root burn.” I disagree.

Epiphytic orchid roots are covered with a relatively thick layer of dead cells, called velamen. The actual root is quite small and located at the center of this structure. The velamen acts just like a sponge and absorbs both water and the nutrients in it. Research has shown that the roots of phals and 10 other species are “almost saturated after 15 seconds and fully saturated after 60 seconds.” Ecologically, this makes a lot of sense. When it rains in nature, the first flush of water through the canopy is the most nutritious and it is important for the orchid capture this.

The loss of water from velamen is much slower and can take an hour or more.

What happens to the absorbed nutrients? It seems as if velamen is able to hold on to charged molecules, such as nutrients. Even when a fertilized root is placed in pure water, the nutrients are very slow to leach into the water. Instead they are held in the root, until they can be properly absorbed and transferred to the leaves.

It takes about 2 hours for the root to absorb all of the water in the velamen.

If you water orchids with water first, the velamen will be quickly saturated with pure water. Subsequent addition of fertilizer will not be absorbed by the roots since the velamen is already full. If you want to fertilize plants, water dry roots with the fertilizer solution and don’t pre-wet them.

Does Bark Media Require Higher Nitrogen Levels?

Some growers have suggested that bark and coco nut husk media requires higher levels of nitrogen than inert media because the microbes decomposing the bark use up the nitrogen before it gets to plant roots.

This idea probably based on the notion that high carbon material, like bark, needs extra nitrogen to decompose – and that is true. It is also true that bark media is covered with microbes that need nitrogen to live. But the idea that extra nitrogen should be used to feed them is a myth.

As discussed above, orchid roots absorb fertilizer water very quickly, long before it is gobbled up by microbes. The velamen also holds on to nutrients once it has them. There is no competition between orchids and microbes during the fertilizing period.

A goal in orchid growing is to prevent the bark from decomposing. You do this by keeping nitrogen levels low, not by keeping them high. Fertilizing with excess nitrogen speeds up decomposition of the media.

Studies have shown that the type of media has a limited effect on orchid growth, and that proper watering, light and nutrient levels are much more important.

Is Flowering Initiated by Withholding Fertilizer?

The most common question from new orchid growers is, “how do I get my plants to bloom again”?  One suggestion is to starve the plant by withholding fertilizer. It then thinks it is in trouble and starts the reproductive phase i.e. flowering.

That is poor advice. Keep fertilizing before and during the flowering period.

Should Fertilizer Include Calcium or Magnesium

Calcium and magnesium are important nutrients and orchids require them in higher amounts than the micronutrients. Orchids get these nutrients from two sources; fertilizer and water.

If you are using tap water it may already contain enough of these nutrients (your municipality can provide these numbers), in which case you should not add more with fertilizer. However, if you are using rain water, or reverse osmosis water, both of which contain few nutrients, it is important to use fertilizer that contains calcium and magnesium.

Both calcium and magnesium precipitate easily and an excess amount shows up as white crust on your media and plant roots, neither of which is good for plants.

What is the correct amount? Calcium at 100 ppm and magnesium at 50 ppm work well (fertilizer with 8% ca and 3% mg).

Does Foliar Feeding Work for Orchids?

Foliar feeding does work for specific situations, certain types of plants and for some of the micronutrients. It is a poor way to feed macronutrients to any plant since the amount of nutrients actually absorbed by leaves is very small. For general plants you can find more about this topic in Foliar Feeding – Does It Work?

Orchids are special plants. The leaves of some types have a very thick waxy coating designed to keep moisture in. If moisture does not travel easily through the leaves, it becomes clear that a foliar feed won’t either. Orchids have fewer stomata than other plants, but contrary to what many believe, stomata are not the entry point for nutrients from a foliar spray. This takes place through micro-pores and it’s unclear to me if orchids have these.

Foliar feeding is used in commercial settings to correct micronutrient deficiencies, but for the home gardener foliar feed is not a good way to fertilize orchids. Apply fertilizer to the roots.

Fertilizing Orchids – Best Practices

Much of the advice I find online is based on one or more of the orchid fertilizer myths discussed above. They are well entrenched in the hobby, but just because lots of people make the suggestion does not mean it is a good one. The following recommendations are based on the above science.

Select a solid water soluble fertilizer that contains micronutrients and has an NPK of around 3-1-3. If you use rain water or RO water make sure it contains calcium and magnesium. If your tap water already supplies these nutrients try to use a fertilizer without them.

Stay away from “organic” fertilizer. These usually have very low levels of nitrogen and orchid media is not a great place for the organic matter to decompose so that your plants can get a feed.

Stay away  from liquid fertilizer because most of your money will be used to ship water around the country. As an example, see Miracle-Gro Plant Food Mist – Huge Fertilizer Ripoff.

Around 60-75% of the nitrogen should be nitrate. The rest can be ammonium or urea.

Make up your fertilizer solution so that you have between 100 and 150 ppm nitrogen.

Water with a fertilizer solution several times a month and use plain water at least once a month. You could fertilize less when plants are dormant, but for home culture it is easier to fertilize year round.

This video will show you how I water and fertilize a small collection of plants. You can see that I don’t control the fertilizer level very well, and I think I will start changing that. When I grew hundreds of orchids, I would make up the fertilizer in barrels and water with a pump and hose.