A selection of some of the houseplant fertilisers I have tried on my plants. Photograph: Jane Perrone.

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From tablets to foliar sprays, choosing what products to use when you fertilise houseplants can seem like a bit of a minefield. Leigh Hunt, the RHS’s principal horticultural advisor, joins me to explain why we need to feed houseplants in the first place, what they need and how to decode those numbers on the back of the packet! Plus I answer a question about aquascaping soil and bring you part four of the On The Ledge sowalong, looking at what to do once your seedlings have emerged.

Here are Leigh Hunt’s top tips on feeding houseplants:

Signs that houseplants need a feed include yellowing leaves and stunted or slow growth, and a lack of flowers on flowering plants.
Following the instructions on the packet may sound obvious, but don’t just assume you know how to use a particular feed!
Some potting mixes come with nutrients mixed in: multipurpose or houseplant compost may come with nutrients to supply the plant for 6-8 weeks. Other potting mixes contain slow-release fertiliser or may be labelled as ‘season-long food; or something similar - these contain slow release fertiliser pellets that give 24 weeks of food. Whichever one you use, do remember to start feeding plants once the nutrients have been used up. The fertiliser pellets can look like slug or snail eggs but the latter tend to come in clusters and break apart when squeezed.
Most houseplants need three main nutrients - nitrogen (N) for general growth, particularly leafy growth - potassium (K2O) aka potash for flowers and fruit - and phosphorous (P2O5) for root growth. All feeds will have those main nutrients in different levels. Certain feeds are high in particular elements, eg citrus feeds contain a higher proportion of nitrogen, for instance.
Often on fertiliser packets you’ll see a set of numbers that refers to the ratio of these three nutrients - eg 30-10-10. The first number is nitrogen, the second potassium/potash, and the third phosphorous.
Rather than focusing on the actual numbers, it’s the ratio that you need to look at (lower numbers probably mean the feed comes ready-diluted). If the numbers are all the same, eg 10-10-10 it’s a balanced feed, but the NPK ratio a feed that concentrates on one nutrient for the benefit of a particular plant type may look very different. For instance, a lawn fertiliser will be strong in nitrogen and be listed as 30-0-4 whereas a tomato feed could be 4-7-10.
Nicronutrients are also important for plant growth. These occur in minute quantities in most fertilisers - the main ones are manganese (Mn), magnesium (Mg), Iron (Fe), molybdenum (Mo), sulphur (S), boron (B), zinc (Zn) and copper (Cu).
There are certain ericaceous plants that do benefit from extra iron and magnesium, so you may opt for a feed like Sequestered iron feed that delivers these micronutrients.
There are many options on the market: if you want a long term feed mixing in slow release granules at the time of planting ins a great plan. Most of the time, you can probably get away with a balanced fertiliser for the majority of houseplants, but take care with orchids which have low nutrient requirements, and with citrus which needs high levels of nutrients.
Organic fertilisers are derived from natural ingredients such as seaweed or comfrey and come with good levels of nutrients and micronutrients. You can make your own (although this can be a bit smelly) which is a great way of keeping your carbon footprint down and save money, or you can buy them. (Personally I use Maxicrop seaweed feed and Ecofective Houseplant Boost.
Wormcasts from a wormery can also be added to the top of pots. Just remember you may end up with some worms in the soil, and be careful to avoid heaping up wormcasts around the base of the plant.
The key time to fertilise is during the growing season for each plant: only feed in the dormant season if there are signs of nutrient deficiency.
Make sure you use the best quality potting soil you can as this will help your plant to thrive and will hold onto nutrients better. Add a quarter of horticultural grit or perlite if the potting mix feels claggy.

Question of the week

Claire got in touch to ask where James Wong sourced his specialist aquarium potting mix from, as she has sourced some bucephalandra and plans to plant them into a smallish vase bought from Ikea. James told me he uses Scaper’s Soil from Dennerle as it’s cheaper than the others but he finds it just as good!

Want to ask me a question about your plant? Email ontheledgepodcast@gmail.com.

On The Ledge Sowalong part four

Thanks to everyone who has been sharing their sowing exploits by posting the hashtag #otlsowalong on Instagram, Facebook and Twitter. Do keep sharing and sowing!

This week I’m looking at how to take care of your young seedlings:

There’s great information on caring for seedlings in this RHS piece.
Once all or almost all of your seedlings in any one tray have germinated, start increasing air circulation by lifting off the propagator lid.
Don’t let seedlings get ‘leggy’ - this happens when they don’t get enough light. You can put them under growlights or make your own homemade light reflector with aluminium foil.
Stroking seedlings will help to strengthen them up by mimicking a breeze.
The key to pricking out seedlings is delicate handling: do not touch the stem: instead lift by the cotyledons or lift their mini-rootballs up from below. Report into a modular cell tray, one per unit, or small individual pots. Use houseplant potting mix rather than seed mix.
What if nothing is happening yet? Patience is all! Don’t worry, some seeds take weeks or months to break dormancy. Large seeds usually have a hard seed coat that takes a while to break down.
Want to grow ferns from spores? Here’s a great guide from the British Pteridological Society.

INTRODUCING THIS WEEK’S SPONSORS…

Always Kalanchoe

This week's episode of On The Ledge is supported by Always Kalanchoe, the website that inspires you to find out about the incredible family of colourful succulents that are just so easy to grow.

There's more to Kalanchoes than you think, from the brightly coloured flaming Katy to the intriguing green-flowered Magic Bells, these little beauties will bloom happily in kitchens, bedrooms, bathrooms, window ledges or outside on patios during the summer months.

As one of the most popular houseplants, they can have amazing health benefits too! From fighting tiredness and increasing concentration to improving air quality, there are plenty of reasons to turn your home into an indoor jungle. And as kalanchoes are one of the most colourful houseplants, they can be a fantastic mood booster too.

Kalanchoes are widely available from florists, supermarkets and garden centres nationwide. Kalanchoe care tips and inspiration can be found on the Always Kalanchoe website - www.kalanchoe.nl/en - or follow them on Instagram @Alwayskalanchoe.

Kiwico

KiwiCo designs hands-on projects delivered to your door for kids aged 0-16+. The great thing about Kiwico is each crate comes with absolutely everything your child needs to complete an awesome, accessible project that makes learning fun - with no need to rush out and buy craft materials. Simply choose your line of crates and get them shipped monthly. You can also cancel or pause your plan at any time.

Kiwico has hundreds of products to choose from, each focusing on different aspects of STEAM - that’s science, technology, engineering, art and maths. Change the way your child plays with Kiwico! Visit kiwico.com/ontheledge and get your first crate free!

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CREDITS

This week's show featured Roll Jordan Roll by the Joy Drops, An Instrument the Boy Called Happy Day Gorkana by Samuel Corwin and Oh Mallory by Josh Woodward. Ad music is by the Heftone Banjo Orchestra: Dill Pickles and Whistling Rufus. All tracks licensed under Creative Commons.

Logo design by Jacqueline Colley.

Episode 90: fertilisers for houseplants plus OTL sowalong part four