Despite what many people say, in most plants potassium does not directly stimulate or 'promote' flowers to develop on plants.

For many years, I worked in a retail nursery and a common question I was asked by customers is about the relationship between the nutrient potassium and flowering. Many people mistakenly believe that if they fertilise their plants with potassium (or "potash"), their plants will produce more flowers.

Unfortunately, there is little scientific evidence which supports the idea of potassium stimulating floral initiation (making more flowers as a result of potassium). It is nothing more than a myth.

This article aims to shed light on the role of potassium in plants and why applying potassium to a healthy plant will not alter the number of flowers that plant will produce.

Let's start by looking at potassium and its role in plants:

Potassium: A macronutrient

Potassium (K) is a common nutrient found in many organic and inorganic fertilisers. It is often called a 'macronutrient' because it is required by plants in high concentrations compared to other nutrients. (Only nitrogen is required in greater amounts than potassium).

Potassium is sometimes sold in the following forms:
  • Potassium chloride (KCl)
  • Potassium dihydrogen orthophosphate (KH2PO4)
  • Potassium nitrate (KNO3)
  • Potassium sulphate (K2SO4)
These compounds are often mixed with other nutrients to make a 'complete fertiliser' as this is often easiest for home gardeners to work with. You can determine how much potassium is in a fertiliser by looking at the N:P:K ratio which is the ratio of nitrogen to phosphorous to potassium.

What does potassium do in plants?

Potassium has an important role regulating water and nutrient movement in plant cells. 

All plants are composed of cells. When you water a plant, water travels from the root cells, through the plant's vascular system (called the xylem) to the leaf and flower cells. Nutrients from the soil also travel a similar path. If lots of nutrients enter a cell, then the cell will have a greater osmotic gradient and want to take in a lot of water and expel the nutrients to restore osmotic balance.

Conversely, if a cell contains a lot of water and few nutrients, it will have a lesser osmotic gradient and will want to take in nutrients and expel the water. This process is called osmosis, and in a natural system an equilibrium would develop so that the osmotic gradient was the same in both cells meaning that there would be the same number of nutrients and water on each side. Plants are not 'natural systems' in this sense because they control their water movement.
Osmotic gradients
Figure 2: In scenario "A", Cell 1 has a high concentration of an orange solute and Cell 2 is a low concentration. If no energy or control were applied (and we assume that all molecules had free passage between cells), the solute will pass from the area of high concentration to the area of low concentration until both cells form an equilibrium (scenario "B"). This process is called osmosis. (This specific scenario is purely theoretical).
So, how do plants transfer water between cells?

A cell cannot just 'grab' water from another cell and likewise water won't just enter a cell for no reason. Plants have to force it to move. Plants transfer water between cells by moving charged nutrient particles called ions which act to change osmotic gradients.

For instance, if a plant wants to transfer water from cell A to cell B, it has to provide an 'incentive' for the water particle to enter the cell. To do this is uses a 'pump'. Potassium ions (K+) are pumped into cell A making it salty. This gives cell A a greater negative osmotic gradient than cell B, and so water moves from B to A until the osmotic gradients are the same again. The more potassium that is pumped, the more water is moved. This is critical to the functioning of stomata; pores in plant leaves which control water loss from the plant.

So potassium is used to regulate water movement between cells.
Potassium pumps
Figure 3: In this scenario, one plant cell is dehydrated and one cell is turgid. Potassium ions (K+) are pumped into cells to create a negative osmotic gradient, thus forcing water to move between cells. This is called a "potassium pump".
Potassium (and some other nutrients) are also used in plant cells to transfer nutrients between cells. Potassium is 'swapped' for other nutrients when cells exchange ions.

Potassium is also required to activate more than 50 plant enzymes which allow critical chemical reactions in the plant to happen (Atwell, Kriedemann & Turnbull 1999) including photosynthesis and respiration (Taiz & Zeiger 1998).

Potassium is also a component in DNA; the plant's genetic code. And in addition to this, potassium promotes cell-wall thickening, thus protecting the plant from disease and strengthening stems.

And finally, potassium can lead to an increase in fruit set in a number of species such as lemon (Quaggio et al. 2002), cotton (Gormus & Yucel 2002) and peppers (Paz et al. 1996) to name a few. Hamlin & Mills (2001) found in pansy (Viola √ówittrockiana) that more potassium was absorbed after the flowers were opened than when they were developing indicating a role in fruit development.

To summarise, potassium:
  • Is used to regulate water and nutrient movement in plant cells.
  • Activates more than 50 key enzymes in plant cells.
  • Is one of the building blocks of DNA.
  • Increases fruit set in some species.
Potassium deficiency

A deficiency of potassium in a plant will prevent stem elongation, so growth is stunted because photosynthesis (sugar production) and respiration (burning of sugars for energy) is limited. This causes leaf yellowing.

Plants may become weak since their cell walls are thinner and this is sometimes seen in addition to the formation of crinkled leaves. Thinner cell walls also results in increased likelihood of pathogenic attack and makes stems weak.

The Link with flowering

You'll notice in this story that flowering has not come up once. If you read a good plant physiology text you'll find the same. That's because potassium has no direct role in promoting flowering in most plants.

Indirectly, if a plant is suffering from a potassium deficiency then growth will be stunted and this stress may limit flowering because the plant cells can't divide to allow the growth. There is no doubt that a potassium deficiency will reduce the quality of flowers.

However, if the plant is not flowering and otherwise looks really healthy (as is the case with most of the cases I have seen) it's not likely to be a potassium deficiency that is the problem. Fertilising with potassium is unlikely to make any contribution towards flower production.

Who says Potassium will stimulate flowering?

I have heard many gardening "experts" state that potassium specifically will promote flowering. This is not just limited to the staff in retail nurseries but horticulturalists who share their 'knowledge' across the airwaves and in newspapers and magazines.
Thrive advertisement
This banner advertisement from the Arthur Yates & Co. website (2004) clearly states that the application of potassium will increase floral initiation, despite there being no scientific evidence.
In a discussion about hibiscus fertiliser, ABC1 Gardening Australia presenter Jerry Coleby-Williams said "The key ingredient is potassium, potash. So, if you use a hibiscus fertiliser, you're guaranteed to get lots of blooms".

Horticulturalist John Patrick, speaking about Debco Phostrogen states that the fertiliser is "a high-potash formula so it's great for encouraging flowering and fruiting". On the same product listing, the site states that the product contains "potash for more flowers and healthier plants".

Arthur Yates & Company state that Thrive‚ĄĘ Flower & Fruit fertiliser is "high in potassium which promotes strong fruiting and flowering".

Better Homes and Gardens writes "Potassium ... is particularly important to the plant's fruiting and flowering".

In an ABC Gardening Australia fact sheet, writing about growing tomatoes, Peter Cundall suggests "many of (the tomatoes) are in flower and starting to form their first fruit, and that is because of the sulphate of potash that has been put around them".

In a rather odd statement about Hoya spp., Neville Passmore wrote "One way to trigger flowering is to supply a high potash fertiliser to the soil and also spray it over the foliage. However, the main reason why hoyas (sic) fail to flower is because they are kept in a position that is too dark." This probably means that light is the trigger for flowering, not potassium.

In the United Kingdom, the Royal Horticultural Society claims "Potassium promotes flowering, fruiting and general hardiness".

There are many more cases of similar advice; far to many to list here.

Of course, it should be noted that there are some rare instances where potassium is thought to have a more direct role in floral initiation such as in mangoes (Protacio 2000). However it is emphasised that potassium is by no means the primary stimulant in even this species.

Conclusion

The key rule of thumb is this: "nutrients do not regulate growth, hormones do". Potassium is not a hormone. It therefore cannot stimulate flowering.

It is important to realise that the process of flowering is extremely complex (as this site will attest) and even a well-qualified horticulturalist of many years experience will not necessarily be able to solve your problem.

In most cases, you can be assured that if your plant is healthy-looking, an ansence of flowers will not be corrected via an application of potassium.