Can ‘bloom’ fertilisers force plants to flower?

I am a hobby grower new to hydroponics. I notice that some hydroponic fertilisers are labelled as ‘bloom”. I am getting conflicting comments as to whether these can force plants to flower. Is this possible?


‘Bloom’ type fertilisers are useful, but do not force the flowering of plants. Plants, which flower start in the vegetative state (that is, growing roots, stems, and leaves). Individual species will have a specific nutrient demand for this stage, which is usually relatively high in nitrogen (N) and low in potassium (K).

When the plant develops and grows into the flowering, and especially fruiting, stage the makeup of its nutrient demand will change. In particular, the relative demand for nitrogen drops and for potassium rises.

‘Grow’ and ‘bloom’ fertilisers

Liquid fertilisers intended for the plant’s vegetative stage are often called ‘Grow’ or ‘Veg’ fertilisers. Those trying to match the nutrient demand of the flowering and fruiting stage are usually called ‘Bloom’ fertilisers. It is important to realise that ‘Bloom’ fertilisers do not initiate flowering. Their aim is to match the nutrient balance uptake by the plant in its flowering/fruiting stage.

Note that, because liquid fertilisers are in a concentrated form, ‘Grow’ and ‘Bloom’ fertilisers will always be supplied in two parts, usually called ‘A’ and ‘B’. The reason for this is that in concentrated solutions the mixture of calcium (Ca) and iron (Fe) ions with phosphate (H2PO4) and sulphate (SO4) ions will be insoluble, so these ions have to be separated. When diluted to the working strength given to the plants all these ions are fully soluble. A rough analogy is that you can dissolve one teaspoonful of sugar in a cup of tea, but not a hundred teaspoonsful in the same cup.

Consequently, ‘Part A’ contains all the calcium nitrate and iron chelate fertilisers and ‘Part B’ contains all the other fertilisers. To even up the concentration of the fertilisers in A and B up to half of the potassium nitrate (which has no solubility problems) is often moved from the B into the A mix.

Flower initiation

There is no standard mechanism for the initiation of flowering. There are numerous different mechanisms, many of which are still uncertain and even unknown, which apply to different species. Sometimes there are even significant differences between varieties within the same species.

Typical influences involved are light level, day length, air and plant temperature, the presence of hormones and other plant chemicals. There can even be an influence from the circadian rhythm within the plant – its ‘biological clock’ if you wish. There are no general rules as to which of these apply to individual species, or even varieties, nor to the degree of influence.

Day length

One of the best-known influences upon flower initiation is day length. Some plants will initiate flowering when the day length gets below a critical number of hours. These are known as ‘Short day’ plants. In fact this terminology is incorrect. They would be more accurately described as ‘Long night’ plants, as this is what initiates flowering. Some plants require only one long night to initiate, whereas others require several weeks.

Other plants are ‘Long day’ plants, initiating flowers when day length exceeds a critical value. Again, the more accurate description would be ‘Short night’ plants. It is possible to shorten the night to control flowering by splitting the night with a period of artificial lighting.

Plants, which don’t respond to day length, are known as ‘Day neutral’.

Whether a particular plant is long day, short day or day neutral is unpredictable and can only be determined in practice.

Other influences

Plants flower in order to reproduce. To do this effectively the plant must be strong enough, that is, it must be mature. Juvenile plants are unable to flower.

It is the leaves, which detect day length for the plant. For flower initiation there must be some form of communication of this to the flower site. For a long time scientists thought that there must be a ‘flowering hormone’ and even gave it the name ‘florigen’. Florigen is now known not to exist, however, other hormones have an involvement. Probably hormone groups such as gibberellins and cytokinins, plus ethylene, sugars, and polyamides interact to cause the induction of flowering. Genes are also involved.

There are no general rules and each plant has different interactions. For example, some hormones have been found to aid flowering in some plants and inhibit it in others. Especially relevant to this question – nutrient manipulation cannot initiate flowering.


Strawberries are interesting because there are some varieties, which are long day, others, which are short day and even others, which are day neutral. Something else about strawberries is that before day length can initiate flowering, the plant must have initiated budding.

This requires that the plant has been exposed to a period of low temperature. This process is known as ‘vernalisation’.


Chrysanthemums are probably the best known plant where day length initiation is used so an entire crop can be aimed to harvest for a special occasion such as Mothers’ Day. Chrysanthemum is a short day (long night) plant, so to keep the plant vegetative the nights are kept short by giving low power lighting for four hours in the middle of the night. Flowering is then initiated for the required date by lengthening the night using blackout screens over the crop.  Ω

PH&G Oct 2016 / Issue 172