Gardeners often kill plants with kindness. 'A little bit’s good, so a lot will be even better' is a sentiment that can lead to overfertilising disaster. We are often heavy-handed with fertilisers, dosing plants up too often or when they don’t need it.
When to feed
As a rule of thumb, most plants only need feeding once a year, and that’s generally best given in spring as growth resumes after winter. It makes sense when you think about it – if a plant isn’t actively growing, it doesn’t need feeding. Indeed, applying fertiliser that’s not being taken up by plants is a waste. It can also lead to excess fertiliser being washed into waterways or otherwise leached away.
Plants that need more frequent fertiliser applications are vegetables (particularly leafy vegies), plants grown for prolific flowers including roses, hibiscus, perennials and annuals, and some fruit trees. Citrus, in particular, benefit from feeding twice year – in late winter (around August) and again in late summer (around February). These are the times when trees are actively growing and developing.
Lawns also benefit from several applications of fertiliser a year, but only if they are growing and that’s usually from spring to early autumn. If it is very dry, very wet or very cold hold off on the fertiliser, particularly on lawns.
Potted plants also need regular applications of fertiliser, but don’t overdo it. Plants that are growing in potting mix that already contains slow-release fertiliser. for example, will not need feeding for three to six months after repotting. Liquid feeding is usually the best option to apply the right amount of fertiliser to potted plants.
How much is too much?
The amount of fertiliser to apply doesn’t have to involve guesswork. All fertiliser containers or packets have recommended application rates, so by reading the recommendations before applying or mixing up a feed, you can avoid overfertilising. Rates may also vary according to the plant being fed, for example ferns only need dilute rates of plant food.
It is also important to keep fertilisers away from direct contact with plant roots. Don’t add fresh manures to planting holes (cover fertiliser with soil or apply to the soil surface) and always water plants well BEFORE applying fertilisers.
An obvious symptom of over-fertilising is browning of leaf edges – they may look as if they’ve been burnt. Plants that have been given high doses of fertiliser – particularly high nitrogen – may develop large, soft leaves that are susceptible to pest and disease attack. Potted plants that are over-fertilised may show evidence of salt build-up on the outside of the pot.
Do's and don’ts of applying fertiliser to your garden
- Don’t overdo it – read the instructions on how much and how often to feed plants before you use any fertiliser.
- Water plants or lawns before applying fertiliser.
- Don’t apply fertiliser when it is extremely hot or cold.
- Avoid applying fertiliser when plants are not actively growing for example during winter or in times of drought.
- Never feed sick plants. Treat the pest or disease problem, apply a seaweed-based plant tonic and hold off applying any fertiliser until there are signs of new growth.