Vitax Q4 vs Chempak No 8 for Flowering Sweet Peas in a 10-Litre Trough

Read the analysis as a release pattern

Vitax Q4 lists 5.3-7.5-10, with added magnesium and trace elements, and it is sold as granules to rake or mix into compost. Chempak No 8 gives 12.5-25-25 and comes as a fully soluble powder, used at roughly 3.5g in 4.5 litres of water. The Q4 granules break down slowly as soil moisture and microbial activity work on them, giving a reserve that lasts about six to eight weeks. Chempak reaches the roots within hours of watering and then disappears quickly through uptake and leaching, so its feeding rhythm is measured in days.

In a 10-litre trough planted with six to eight sweet pea plants, the first number on the label can draw too much attention. Sweet peas are legumes. Once established, their roots work with Rhizobium bacteria that fix atmospheric nitrogen. Extra nitrogen can push leaves and tendrils ahead of flower buds, especially in the confined root run of a trough.

The third number, potash, matters more once flowering begins. Chempak No 8 carries 25 percent potash, while Q4 carries 10 percent. That gap explains why Chempak No 8 exists as a separate product in the Chempak range.

Potassium helps regulate the movement of sugars and water inside the plant. It also influences how many buds a sweet pea sets and carries on with, instead of aborting them. A plant under water stress in a small compost volume often sheds buds first. Higher potassium availability helps buffer that response during the flowering stage.

That is why Chempak No 8 fits the moment when buds are forming quickly. Sutton Seeds and heritage sweet pea suppliers such as Owl’s Acre also push a high-potash, tomato-style feed once flowering starts. The advice lines up with the same plant demand: enough potassium available while stems and buds are being produced.

Q4 is still useful. In open ground, where a sweet pea has a larger root run and soil biology keeps nutrients moving, 10 percent potash can be adequate. A trough behaves differently. Its reserve is small, and daily summer watering can wash dissolved nutrients through the drainage holes. Even a litre of water moving through 10 litres of peat-free compost carries some nutrient away. Q4 keeps releasing, yet the pace is governed by warmth and breakdown in the compost instead of the plant’s peak flowering demand.

During a hot fortnight, when the plants are throwing stems fastest, Q4 alone cannot raise the potash concentration in the compost quickly. Many growers therefore use the products in sequence: Q4 mixed into the compost at planting as the base charge, roughly 100g for a 10-litre trough, followed by Chempak No 8 every 7 to 10 days after the first flower buds show colour. The base feed covers establishment. The soluble feed covers the surge in bloom.

A schedule for one trough

At planting in April or May, rake 100g of Vitax Q4 into the top third of the compost. That gives a starting NPK reserve for the six-to-eight-week establishment phase, when the plants are building roots and stems before the main flowering period.

Once buds colour up, mix Chempak No 8 at label strength: about 3.5g in 4.5 litres of water. A batch of that size is convenient for a small trough because it can be used across watering sessions without needing tiny pinches of powder each time. Feed on about every seventh watering, with plain water between feeds. In warm weather, that normally settles into one liquid feed every 7 to 10 days.

That plain-water gap matters in a container. Soluble fertiliser leaves salts behind if the compost is repeatedly fed while evaporation is high and drainage is imperfect. Keep alternating feed and plain water so the electrical conductivity around the fine feeder roots does not rise to scorch levels.

Across a 12-week flowering run from mid-June to early September, the trough receives about 10 to 12 liquid feeds. A 750g tub of Chempak No 8 makes roughly 950 litres of feed solution at label strength, enough for several troughs across a full season with room left over. The soluble-feed cost per trough comes in under £2 for the season, while the Q4 base charge adds about 50p.

The thing feed will not fix

A sweet pea that stops flowering in August is often carrying seed pods. A maturing pod signals the plant to slow bloom production, and fertiliser does not override that signal; deadheading before pods swell is what keeps the 12-week cutting run going.

Label wording that causes poor choices

The front of a Chempak No 8 pack calls it a flower feed, and Q4 is sold as a complete plant food. Both descriptions are accurate in a narrow sense. Complete means it contains the three main macronutrients in some ratio. Flower feed usually means a high-potash formulation.

Those phrases do little work in a trough because they say nothing about release rate. That property separates the products in a small container. One supplies a slow reserve, the other supplies a short, soluble pulse.

Dose advice can also drift from open borders into troughs. A feed strength suited to a plant in a full flower bed is diluted through a large soil volume. The same dose poured into 10 litres of compost becomes more concentrated around the roots.

Peat-free composts, now the default since the retail peat ban rolled out across the UK, add more variation. Coir and bark-based mixes hold, release and bind potassium at rates that differ from batch to batch. A fixed 7-day interval on a Chempak carton is a starting point for a warm, fast-draining trough, and the weather may shorten the practical interval. The label cannot know the compost mix or the week ahead, so it prints the cautious middle case and calls it a schedule.

The nitrogen figures need the same reading. A higher first number looks like more food, and for a leafy plant it can be. With sweet peas, the root nodules supply much of the nitrogen once the plant is established. Q4’s 5.3 against Chempak’s 12.5 does not make Q4 the weaker flowering feed, because flowering is being driven by potassium availability.

Keeping the trough through winter

A 10-litre plastic trough left outside in a hard frost can crack at the corners as wet compost expands. Terracotta troughs split more readily still. If the same trough carries autumn-sown sweet peas through winter for an early crop, it needs to shed water freely.

Raise the container on pot feet so the drainage holes never sit in standing water. Moving it against a house wall can keep the overnight low a degree or two above the open garden. Roots left in cold, saturated compost rot before spring, and feeding cannot restore a plant that has lost its root system to January waterlogging.

Autumn-sown plants should go through the coldest months without feed. Growth is near zero, uptake is minimal, and soluble fertiliser applied in November either washes through or accumulates as unused salt. If Q4 is being used, the base charge can wait until growth restarts in March. Chempak No 8 has no role until buds appear the following June.

When convenience gives Q4 the job

For flowering performance in a 10-litre trough, Chempak No 8 supplies what Q4 cannot supply quickly enough: 25 percent potash in soluble form at the point when buds are being set fastest. Q4 belongs at the establishment stage, mixed into the compost before flowering begins.

Gardeners who cannot keep a 7-to-10-day liquid-feeding rhythm through summer have a different problem. Someone away for stretches, or using an automated system that delivers only plain water, is usually better served by Q4 alone at a heavier base charge, accepting fewer stems in exchange for a trough that is not dependent on regular hand feeding.

Automated plain-water irrigation exposes the practical gap between the two feeds. It can keep moisture moving through the trough, while the soluble flower feed still depends on someone being present. The awkward mismatch is absence during the exact spell when sweet peas ask for potash most sharply.