Fertilisers for window boxes and potted plants: when and how to use them?

How to choose fertiliser for planters and pots?

Naturally, you expect your annual, biennial or perennial plants, grown in planters, hanging baskets or pots, to offer you beautiful, abundant and colourful blooms throughout summer.

Why add a fertiliser to balcony plants?

But, at the same time, these plants grow under sometimes harsh conditions, constrained by the small size of their container and often full-sun exposure. Indeed, these geraniums, petunias, lobelias, annual verbenas and other bacopas expend a great deal of energy to flower. They therefore need nutrients to deliver the display. Much more than the same plants grown in the ground.

D’une part, les plantes fleuries cultivées dans des contenants bénéficient de beaucoup moins de substrat. Donc de moins de minéraux et d’oligo-éléments à leur disposition. D’autre part, ce même substrat, proposé en quantité limité, s’épuise très vite. Tout simplement à cause des arrosages ou de la pluie. En effet, l’excédent de l’eau apportée aux jardinières, potées et suspensions, a tendance à s’échapper par les trous de drainage, entraînant avec elle les nutriments. Alors que, en pleine terre, l’eau, chargée en éléments nutritifs, remonte par capillarité pour nourrir les plantes. Et, comme globalement les plantes sur les balcons et les terrasses sont arrosées plusieurs fois par semaine, les nutriments fuient en masse.

C’est pourquoi il est essentiel de faire des apports réguliers d’engrais pour obtenir une belle croissance, mais surtout des floraisons resplendissantes. Mais pas n’importe quel engrais !

In pots or planters, the growing medium is quickly exhausted. Fertiliser helps nourish plants.

Which fertilisers are suitable for plants in planters and pots?

For flowering plants grown in pots, prefer fertilisers high in phosphorus (P) and potassium (K), two primary macronutrients useful for root system development and flower formation. Nitrogen, by contrast, contributes more to growth of the aerial parts of the plant, namely stems, leaves and buds. However, if your planters contain flowering annuals and perennials chosen for their foliage (Coleus, heuchera, hosta…), nitrogen is also necessary.

That is why, for your flowering planters and hanging baskets, you should select fertilisers with an NPK formulation in which potassium (K) has the highest number. Some are even enriched with trace elements such as boron, copper, iron… to correct common deficiencies. Or with magnesium, necessary for development of attractive, green foliage. In any case, these fertilisers are clearly identified as specially suited to balcony flowering plants such as geraniums.

Different forms of fertiliser and their uses

You now know which fertiliser is best for your plants in planters, hanging baskets and pots. But a new dilemma presents itself. Organic, mineral or organo‑mineral fertilisers are available in three particular forms: granules or powders, sticks and liquid. So which to choose between solid fertilisers or soluble ones?

• Granules or powder fertilisers: these are slow‑release fertilisers that will release nutrients over the long term, roughly the three months of full flowering. Most often of organic origin, these solid fertilisers need time to break down and release nutrients. They therefore help prevent potential over‑fertilisation, but they require a little preparation to integrate into the substrate.
• Fertiliser sticks: these are also slow‑release solid fertilisers, perfectly suited to flowering plants grown in pots, hanging baskets or planters. Some are specifically designed for houseplants. Very easy to use and no risk of over‑fertilisation.
• Liquid fertilisers: they are diluted in watering water for an immediate pick‑me‑up. They are instant‑release fertilisers also well suited to potted plants. Nutrients are therefore very quickly assimilable. These fertilisers are very easy to use, but there is always a risk of over‑fertilisation if doses and frequency of application are not respected.
  

  There are three types of fertiliser for planters and pots: granules or powder, sticks or liquid.

Practically speaking, for plants in planters, pots or hanging baskets, we recommend using a slow‑release solid fertiliser and a liquid fertiliser for a boost. Indeed, the needs of these plants are greatly increased and both types of fertiliser will be necessary to meet them. Without this feeding, your annual or perennial plants may languish and fail to flower as they otherwise would.

When and how to apply them?

Now consider method and timing of application. As noted above, to ensure good health and flowering of plants in planters and pots, using two different fertilisers is recommended.

Slow-release fertilisers, whether in powder, granules or sticks, are applied at planting in spring. This allows nutrients to be released to plants over a long period, gradually. Since this type of fertiliser releases over around two to three months, it is possible to make a further application during summer, in August or September to extend flowering. For dosages, refer to instructions on brand labels. Above all, adhere to recommended doses to avoid deficiencies or causing adverse effects.

At planting, simply mix the solid fertiliser into the substrate. If adding during cropping, spread powder or granules over potting substrate, then lightly fork in. Then water well to dissolve fertiliser.

Container-grown plants need regular fertiliser applications.

Fertiliser sticks are easier to use: simply push them into potting compost. Nutrients will be released progressively with each watering.

Fast-acting or soluble fertilisers are used during watering in the growing season, roughly April to September, depending on varieties of annual or perennial. Dosages and frequency vary by brand; these details are clearly explained on packaging.

Simply mix capful contents into watering can water. For better dilution, lightly moisten growing medium before watering with fertiliser solution.