After watering, feeding is the most important task you’ll need to perform for vegetables grown in containers. While plants in the open ground can send out roots to seek out additional nutrients once they’ve used up those nearby, container plants are effectively quarantined from the nutrients, fungi and bacteria naturally found in soil.
This means that, before long, they will need a helping hand from you, the gardener.
How to Fertilise a Container Garden
Whether you’re mixing your own potting compost or using a proprietary brand, likelihood is that plants will exhaust all its available nutrients within about six weeks. Mixing in a granular slow-release organic fertiliser or some chicken manure pellets will help to give an extra boost later on.
But even that won’t be enough for hungry container plants. A regular liquid feed will become necessary, particularly for greedy vegetables like brassicas or tomatoes (yes, I know tomatoes are really fruits!). You can buy liquid feeds or make your own. Diluted with water, they provide a shot of extra nutrients that ensures plants continue to grow well and produce yields to be proud of.
Using Liquid Fertiliser in Your Container Garden
There are many types of liquid fertiliser on the market. Look for the N-P-K ratio on the back of the pack and, for most crops, choose one that has an approximately equal amount of each. N stands for nitrogen, P for Phosphorus, and K for potassium, which are the three nutrients that plants need most of.
For fruiting plants however, choose one with a higher K number. Potassium is required to help tomatoes, peppers and other fruiting plants flower and produce their fruits, so it makes no sense to skimp on this. Follow the manufacturer’s instructions on how much to feed; too much can be harmful, while too little will be ineffective.
The liquid feed that I use most often in my garden is made from seaweed. Technically a plant growth stimulant, rather than a true fertiliser, it promotes the general health of plants. I like to use a weak seaweed solution when potting on or transplanting, as it seems to keep plants looking perky after this minor trauma. I also water all my vegetables with a dilute seaweed feed about once a month. If plants are looking a bit under the weather, watering or spraying a seaweed solution directly onto the leaves will often sort them out.
Make Your Own Liquid Fertiliser
Filling your shed with bottles of liquid feed can become pricey, so consider making your own. Beware though – the stink is legendary! Comfrey is the most commonly homemade liquid fertiliser. It’s great for fruiting vegetables because it contains a good dose of potassium – check out our article on making comfrey fertiliser for directions on how to grow and make fertiliser from this indispensible herb. Nettles or borage can be used in the same way for a higher-nitrogen alternative, which is beneficial for leafy vegetables.
A fertiliser ‘tea’ can also be made by soaking a bag of compost or well-rotted manure (from a trusted source) in a large bucket of water. Ten days later, dilute the dark brew with more water until it’s about the same colour as weak tea then use to feed your plants.
Or perhaps you’d like to try making a urine-based fertiliser. Urine is perhaps not the first source of nutrients you’d consider, but it’s sterile, high in nitrogen and contains a useful amount of potassium, together with other nutrients. Dilute it extremely well – up to 40 parts water to one part urine – and don’t let anyone see you, ahem, extracting it from the source!
How Often to Fertilise Container Vegetables
As I mentioned above, a monthly seaweed feed helps keep most container vegetables in good health. Fruiting vegetables will need a tomato feed weekly (alternating with the seaweed feed once a month). Fertilise throughout the growing season from spring until late summer.
Some container plants should not be fed. It’s tempting to give stressed plants a liquid fertiliser as a pick-me-up, but plain water is best until they’ve recovered. Cut-and-come-again lettuces or other salad leaves don’t typically need a regular feed. Herbs shouldn’t need to be fed at all, particularly lavender, thyme or rosemary – they do best in nutrient-poor, drier conditions.