How and when to fertilize cucumbers – for a bounty of fruits each summer

Regular feeding is important to keep this hungry crop producing lots of tasty cucumbers

Cucumbers growing on plants
(Image credit: Getty/Andrei Potorochin)

Cucumbers are hungry crops that need lots of water and feed to ensure they produce a bumper crop of fruits. If you want a bounty of cucumbers for salads or sandwiches throughout the summer, then regular fertilizing is a vital part of maintenance.

When growing cucumbers it is crucial to give them a feed at the time of planting and then when they start producing flowers. Regular fertilizing will help them keep on producing flowers and ripening fruits.

We take a look at the most important times to apply fertilizers to cucumber plants and the best feeds to use to help guarantee you a fantastic crop.

Cucumbers growing on the plant

Cucumbers are warmth-loving plants and predominantly climbers

(Image credit: Future)

When to start fertilizing cucumbers

Cucumbers are commonly sown indoors as part of greenhouse planning, ready to be planted into their final position once the risk of frosts has passed. There are always enough nutrients in seed compost to start the seedlings off, but there are benefits to adding some fertilizer to the soil when plants are potted on. 

The time for when to fertilize seedlings is once they get to around six inches tall and need moving into larger pots to grow on. Adding a slow-release fertilizer mixed in with compost can help give the seedling a boost and provide it with the nutrients required until it is large enough to be planted in the greenhouse, vegetable garden, or as part of a container garden.

While the addition of fertilizer at the seedling stage is not always vital, the same cannot be said of when it comes to planting cucumbers. At this time it is crucial, and there are a few options available to the grower. 

You can incorporate compost or well-rotted manure into the planting site, which has all the vital nutrients plants need for strong and healthy growth that it releases over an extended period of time. The other option is to add some slow-release and balanced fertilizer into the planting site to give plants a good boost of nutrients to get them started in their final planting spots. Cucumbers are often seen as a great crop to grow in a greenhouse or people grow cucumbers vertically over arches or trellises as part of vertical gardening ideas to make the most of space.

The next point in time where fertilizing comes to the fore is when the cucumber plants start to show their first flowers. At this stage, the plants need lots of potassium, which is the nutrient vital for flowering and fruiting and it is required to ensure you can pick cucumbers regularly over the season.

John Negus, a gardening expert for Homes & Gardens, recommends that in June after the frosts finish, growers transplant their cucumber seedlings ‘into sunny, well-drained and fertile soil, enriched with composted manure’. 

He adds: ‘Liquid-feed indoor and outdoor plants weekly with a high-potash tomato fertilizer’.

John Negus
John Negus

John has been a garden journalist for over 50 years and regularly answers readers' questions in Amateur Gardening magazine. He has also written four books and has delivered many talks over the years on horticulture.

Cucumber flower and small fruit

Cucumbers benefit from a potash feed when they start flowering

(Image credit: Getty/rbkomar)

When to fertilize cucumbers in pots

Cucumbers are crops that are well-suited to growing in vegetable garden containers, while they are also commonly grown in grow bags both indoors and outdoors. It is important to incorporate fertilizer into any potting mix, as cucumbers are heavy feeders that can quickly use up all the nutrients in the finite amount of soil in the container. 

Therefore, feeding becomes vital during the growing season to make sure plants have all the nutrients required to grow and produce fruit. A lack of nutrients can cause many problems, including cucumber leaves turning yellow and plants not producing as much fruit as you might have hoped.

Ruth Hayes, a gardening expert for Homes & Gardens, describes cucumbers, as well as the likes of tomatoes, zucchini, and squashes, as ‘super-feeders’ that can easily use up any goodness in the soil quickly when grown in containers or grow bags. 

She recommends: ‘To get the best results, feed and water them regularly. When it comes to feeding, ‘little and often’ is better than a big feed once a fortnight. A weekly feed with a liquid, high-potassium fertilizer will encourage good flowering and fruiting. Don’t let the compost dehydrate as this will inhibit the plants’ take-up of feed.’

Ruth Hayes

Ruth is the gardening editor of Amateur Gardening magazine and is horticulturally trained, with a qualification from the Royal Horticultural Society. Her work for Homes and Gardens and Amateur Gardening, the world's oldest weekly gardening publication, involves matching gardening tasks with each season.

Cucumbers hanging from a stem in a sunny garden

Cucumbers crop from mid-summer onwards 

(Image credit: GettyImages)

What fertilizer to use for cucumbers

It is important to understand plant fertilizer numbers that appear on the packaging to outline the make-up of nitrogen, phosphorus and potassium, referred to as NPK. These show as three numbers, with a balanced fertilizer showing a ratio such as 5-5-5 or 10-10-10. The ratio explains how much of each nutrient is contained in the feed and helps to make decisions about which fertilizer to use. It is vital to always use fertilizers at the manufacturer’s recommended rate and never exceed them.

When planting cucumbers, compost or well-rotted manure are great choices to incorporate into the soil. Or add a handful of balanced slow-release granular fertilizer to the soil. A great option for a fertilizer that can be used at the planting stage is the Expert Gardener All Purpose Plant Fertilizer 10-10-10 available at Walmart.

Once the cucumber plants are flowering, then a fertilizer designed for growing tomatoes is the best option for a feed that is high in potassium. Potassium is a huge boost in helping plants produce flowers and form fruits. Such feeds are commonly used in liquid form, being mixed with water to be added when watering plants. A high potassium fertilizer will display a high third number on the label. As well as tomato feed, other high-potassium fertilizers to consider include using wood ash or kelp meal. An example of a liquid tomato feed to use is the Great Big Tomatoes Fertilizer, available on Amazon

There are natural fertilizers that can be utilized to feed cucumbers. These include making comfrey fertilizer or feed from nettles. These natural plant fertilizers are simple to make and contain all the vital nutrients required by plants. By cutting the plant’s leaves and mixing in water, you make a nutrient-rich tea that needs to be diluted and applied to plants. 

Cucumbers can be simple plants to grow. Watering and fertilizing are essential tasks to regularly complete, while you can also prune cucumber plants to keep them at their most productive. Many growers also utilize cucumber companion planting to help combat pests or diseases that might want to hamper plants and to help them maximize their yield.

Drew Swainston
Content Editor

Drew’s passion for gardening started with growing vegetables and salad in raised beds in a small urban terrace garden. He has gone on to work as a professional gardener in historic gardens across the UK and also specialise as a kitchen gardener growing vegetables, fruit, herbs, and cut flowers. That passion for growing extends to being an allotmenteer, garden blogger, and producing how-to gardening guides for websites. Drew was shortlisted in the New Talent of the Year award at the 2023 Garden Media Guild Awards.