Blossom End Rot on Sungold Tomatoes Corrected with a Vitax Calcium Feed

Sungold tomatoes show blossom end rot when calcium fails to arrive at the far end of a fast-swelling fruit. The Vitax calcium feed corrected the sunken brown patches on the trusses within about three weeks, and the timing made the bottle look like the whole explanation. In most UK garden soils, and in almost every peat-free or loam-based compost sold in 40 litre bags, calcium is already present in quantities far beyond the tomato plant’s total demand. The mineral was available all along. The feed was applied with a new routine, and liquid calcium products are normally used on compost that has first been brought back into a workable moisture range.

Blossom end rot is a transport failure inside the plant. Calcium moves mainly in the transpiration stream, drawn upward as water evaporates from the leaves. Once calcium has reached a tissue, it has little freedom to move again, so new tissue depends on a constant incoming supply. The blossom end of a swelling fruit is at the end of that route, and during rapid cell division it is easily deprived. Sungold, as a cordon cherry tomato carrying dense trusses and filling its fruit quickly, has less margin for interruption than a slower-swelling beefsteak.

Why Sungold Reacts So Quickly to Dry Compost

A single Sungold truss can carry fifteen to twenty fruit. Each fruit is expanding on the same root system, often under glass where heat and light drive growth hard. That speed turns a small lapse in moisture into visible damage. When compost moves from saturated to dry and back again, the plant cannot maintain the steady pull of water that carries calcium to the fruit tip. Cells at the blossom end, short of calcium while they are dividing, break down into the familiar leathery brown crater.

Container plants and grow-bag plants suffer most because their root volume is small. Under glass, a 10 litre pot can move from adequately moist to dry in a single warm afternoon. Border soil gives the roots a wider moisture buffer; a small pot gives them almost none. The practical correction is to keep the compost from falling below field capacity. Little-and-often watering, sometimes twice daily in a July greenhouse, gives the plant a steadier stream than a large weekly soak that leaves long dry gaps between waterings.

High air temperature adds pressure. Leaves transpire faster, roots struggle to supply the flow, and the leaves take priority in the calcium stream. The fruit then loses out. Shading the glass with Coolglass or a fitted net during the hottest weeks lowers leaf demand and leaves more of the flow available for the trusses.

What a Vitax Feed Can Change

Vitax and similar calcium supplements supply calcium nitrate or chelated calcium in solution. A tomato plant can take some up through the roots, and foliar feeding can add a trace through the leaves. Fruit already showing blossom end rot will not recover, so the visible gain appears on the next truss up. That improvement often follows the new watering discipline that begins when the bottle is opened.

The nitrate part of the product deserves attention. A strong nitrogen push produces leafy growth, and extra leaf growth intensifies the competition between leaves and fruit for calcium. A balanced tomato feed such as Tomorite has a higher potassium ratio, which steers the plant toward fruiting growth and can ease the calcium bottleneck. Used with a potash-led tomato feed, a calcium supplement can fit into a regime that supports fruit development more cleanly than either product used alone.

Magnesium can blur the diagnosis. Epsom salts sprayed onto yellowing leaves compete with calcium for uptake at the root. A heavy magnesium habit can therefore worsen blossom end rot while making the foliage look greener. Where Epsom salts are already being used fortnightly, that routine is worth pausing before adding more calcium to the schedule.

Neudorff Fungus Spray Has No Role in This Disorder

Blossom end rot is a physiological disorder, not an infection, so Neudorff fungus spray will do nothing for the brown sunken patches on fruit. That product belongs with problems such as grey botrytis fuzz on a wounded stem or pale leaf mould spotting on older foliage, both of which like the same humid greenhouse conditions that can accompany the rot.

Building a Water Reserve Around the Roots

Advice to water evenly is accurate, yet it leaves the hardest garden problem untouched: a greenhouse can reach 35 degrees by noon, and small volumes of compost cannot buffer that heat for long. The practical answer sits in the growing medium, the container size, and the way water is held near the roots.

A grow bag may hold only 8 to 10 litres of usable water. It also dries unevenly, with the ends becoming parched while the centre remains wet. Cutting the bag into one long trough gives roots a more continuous run. Another method is to stand three plants in bottomless pots pushed into the bag, which creates a deeper rooting zone and slows the dry-down. Ring culture, an older technique, uses the same principle: the plant sits in a collar of compost above a gravel or capillary base, with moisture wicked upward as the roots need it.

Automatic drip lines remove much of the human variability. A Hozelock or Gardena kit on a timer, set for two short bursts a day, keeps moisture moving into the root zone before the compost swings hard toward dryness. Capillary matting under pots gives a slower, gentler version of the same effect at a lower cost.

Border-grown tomatoes still benefit from surface protection. A mulch of grass clippings or spent compost slows evaporation from the top few centimetres, where the finest feeding roots sit. During a hot spell, that surface cover can halve the frequency of watering needed to keep the compost inside its comfortable moisture band.

Rainwater adds another small advantage. A diverter on the greenhouse downpipe can fill a butt with soft water at ambient temperature. Cold mains water poured into hot compost can check the roots briefly, and even a brief root check interrupts the flow that carries calcium toward the fruit tip.

The least glamorous repair is potting on. A Sungold in a 30 litre tub with a proper reservoir base rarely shows blossom end rot, because the larger mass of moist compost absorbs daily swings that a grow bag cannot smooth out.

Reading the Pattern on the Truss

The first fruit of the season and the lowest fruit on a truss show blossom end rot most often. Later trusses may come clean without any intervention as the plant settles, the season warms, and the root system becomes more established. That timing can make almost any spray or feed look successful if it was applied just before the plant corrected itself.

Persistent rot, appearing truss after truss, points to the watering regime or to a root system that cannot keep pace with fruit growth. Digging a finger into the compost before watering gives useful information: the target is the band between wet and dry, with water added before the pot slips too far down that scale. A cheap moisture meter pushed to the base of the pot gives the same reading with less guesswork, and a Sungold under glass needs catching before the compost is halfway to dry.

The unresolved part is visible only in the next clean truss: whether the earlier damage came from a passing early-season check or from a water reserve still too small for the plant.