Stop reaching for that tomato feed. August is the critical month when nitrogen-rich fertilisers become your enemy, not your ally. Your plants are pouring energy into ripening fruit, not making leaves, and the wrong nutrient balance now will steal flavour, delay harvests, and waste your entire summer’s work. Here’s what to feed instead.
By early August, tomato plants across the UK and US have shifted into their final fruiting phase. The plants have stopped their vigorous vegetative growth, and those heavy trusses of green fruit are swelling fast. What many gardeners don’t realise is that continuing with nitrogen-heavy feeds at this stage actively works against ripening.
On cordon varieties, pinch out the growing tip two leaves above the top truss so the plant sets no further fruit it cannot ripen.
Dark blue-green, lush foliage on a fruiting plant often indicates excess nitrogen, but accompanying downward leaf curl is non-specific and may instead reflect water stress, herbicide damage, virus, or other physiological causes.
Keep watering steady once feeding eases off, because erratic moisture splits swelling fruit and triggers blossom end rot.
Nitrogen promotes leaf and stem growth. Feed your tomatoes with high-nitrogen fertiliser in August and you’ll encourage the plant to keep producing foliage instead of channelling energy into fruit development and sugar accumulation. The result is green fruit that stays green, smaller harvests, and tomatoes that taste watery or bland when they finally ripen in September.
Research from university extension services suggests that excessive nitrogen at this stage can extend the growing season by up to two weeks, pushing ripening dangerously close to the first autumn frosts. In cooler regions across northern England and the northern US states, this timing shift can mean the difference between a full crop and a partial loss to frost.
Once the trusses are swelling, each nutrient plays a very different role. This table sets out what to drop, what to step up, and how often to apply it in the final fruiting phase.
| Nutrient or feed | What it does | Late-season action | Frequency |
|---|---|---|---|
| Nitrogen | Drives leaf and stem growth at the expense of fruit | Stop completely from late summer | None |
| Potassium sulphate (sulphate of potash) | Accelerates sugar accumulation and strengthens immunity against late-season disease | Make it the main feed from late summer | Every 10-14 days |
| Phosphorus (bone meal or fruit-ripening feed) | Supports the final stages of fruit development | One low dose in late summer | Single application |
| All feeding | Nothing further once fruit is colouring; surplus is wasted | Stop when most fruit has begun to colour, late summer to early autumn | None |
In short, swap growth nutrients for ripening nutrients, then taper off altogether.
Switch your feeding strategy immediately with these four actions:
Watch your fruit closely over the coming weeks. You should see a visible shift in ripening pace within 7-10 days of stopping nitrogen feeds. Coloured fruit will deepen, and green tomatoes will begin their colour transition. Keep an eye on the weather forecast from mid-September onwards, particularly if you’re growing outdoors in cooler zones, and be ready to harvest any remaining green fruit before the first hard frost.
Remove the leaves below the lowest ripening truss to improve airflow around the fruit.
If brown, water-soaked blotches of blight appear on leaves or stems, pick every sound fruit at once and ripen it indoors.
Green tomatoes ripen faster shut in a box or paper bag with a ripe banana, whose ethylene triggers the colour change.
