When should I use a high-potash feed, and on what?

The single most useful thing to understand about feeding is that plants want different things at different points in their year, and one bottle covers most of the second half. Nitrogen builds the plant; potash makes it flower, fruit and toughen up. Switching between them at the right moment is most of what feeding well amounts to.

Safety

What you'll need

• A high-potash liquid feed, or tomato feed
• A watering can
• A measuring cap

Step by step

1. Recognise a high-potash feed on the shelf.The third number is substantially larger than the first — typically something like 4-3-8, 3-1-5 or 2-1-4. Products labelled tomato feed, rose feed, flower feed and fruit feed are almost all variations on the same thing.
2. Switch at first flower bud, not at planting.This is the timing that matters. While a plant is building leaves and stems it wants nitrogen. Once it starts setting buds, potash is what it needs, and feeding nitrogen at that point gives you a large leafy plant with little fruit.
3. Use it on everything grown for flowers or fruit.Tomatoes, peppers, chillies, aubergines, cucumbers, courgettes and squash. Strawberries, raspberries, currants, gooseberries and blueberries. Roses, dahlias, sweet peas, fuchsias, pelargoniums, hanging baskets and container bedding. Citrus in summer.
4. Feed weekly during the cropping season.Weekly at the stated dilution is the standard interval for a liquid feed in containers, because it leaches out within a week or two of watering. Twice-weekly at half strength suits heavy croppers such as tomatoes in a growbag.
5. Water first, then feed.Never onto dry compost. Fertiliser concentrated around dry roots burns them, and dry compost in a pot channels the liquid straight down the sides and out. Water, wait, then feed.
6. Use it in late summer to ripen wood.Potash hardens growth and improves frost resistance. A high-potash feed on wisteria, on newly planted shrubs and on tender perennials in August and early September helps them harden off, at exactly the point when nitrogen would be actively harmful.
7. Do not use it on leafy crops.Lettuce, spinach, chard, cabbage and kale are grown for their leaves and want nitrogen throughout. High-potash feed on them is money wasted and slows growth.
8. Do not use it on a plant that is still establishing.A newly planted shrub, a young tree or a plant that has just been potted on needs to make roots and leaves first. Feed balanced, or not at all, until it is growing away.
9. Stop by early autumn.Most container plants stop responding as growth slows. Feeding into October does nothing useful and leaves salts in the compost.

Tips

• Tomato feed is usually the cheapest high-potash liquid on the shelf, and it is chemically the same thing as products sold at three times the price for roses or for flowering shrubs.
• Comfrey liquid made at home is a genuine high-potash feed and costs nothing. It smells strongly and needs diluting to the colour of weak tea.
• Sulphate of potash is the dry equivalent — useful applied around fruit bushes and roses in late winter as a slow-acting base dressing, ahead of the liquid feeding season.
• Wood ash from clean untreated wood supplies potassium, along with a substantial liming effect. Useful around fruit on acid soil, unhelpful on chalk.
• Plants in the open ground rarely need a potash feed at all if the soil is decent — this is largely a container and heavy-cropping regime.

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