10 Kitchen-Cupboard Garden Hacks, Fact-Checked

Kitchen-cupboard garden hacks travel by repetition, not evidence. Checked against RHS, Garden Organic and university research, the picture is mixed: coffee grounds are useful in compost but do not reliably acidify soil or deter slugs, eggshells and Epsom salts do not fix blossom end rot, eggshells failed an RHS slug barrier trial, and washing-up liquid kills aphids but carries a higher risk of plant injury than a proper soap.

Quick reference

Coffee grounds

On soil. Coffee grounds contain nitrogen, phosphorus, potassium and magnesium, so they are useful for soil enrichment, and Washington State University found coffee grounds used as mulches or amendments have mostly positive effects on soil. The acidifying claim fails: studies of decomposing grounds have found pH from mildly acidic to somewhat alkaline, the pH is not stable, and any change is only in the immediate vicinity of the grounds. Caffeine left in the grounds can reduce plant growth by suppressing root and shoot growth. Add grounds to the compost heap, or lay them on top as a thin mulch rather than working them in (no more than half an inch of composted grounds as a mulch, because a thick layer compacts and impedes water and air). Fresh grounds are phytotoxic to a variety of plants, so compost them first and keep them to no more than 20 per cent by volume of a compost heap. Scotland's soils tend to be more acidic than the rest of the UK already, so see soil pH testing and adjustment before trying to shift pH. For blueberries and other acid-loving plants, use ericaceous compost, or test the soil and use elemental sulphur as directed.

Against slugs. There is no published evidence that coffee grounds repel or kill any garden pest.

Eggshells

Against slugs. A recent RHS study in a garden-realistic scenario found no reduction in slug damage from barriers made of copper tape, bark, eggshells, sharp grit or wool pellets.

For blossom end rot. Blossom end rot is a local lack of calcium in the fruit, but it is normally caused by uneven water supply, not by missing calcium in the soil: it is very rare for soils, growing bags or potting media to lack calcium. Eggshells do not fix it, and it is best not to guess whether your soil has enough calcium: do a soil test and keep the soil moisture even, especially during flowering (Mississippi State University). Fruits are far from the roots and need a good flow of water to receive calcium, and plants in pots and growing bags are most at risk of an irregular supply. Fertiliser added to dry soil can make it worse, and so can ammonium salts, which compete with calcium.

Epsom salts

Epsom salts are magnesium sulphate and contain no calcium, so they cannot treat a calcium problem. Adding magnesium may create more rot, because magnesium and calcium ions compete for uptake (North Dakota State University Extension), and most soils already have plenty of calcium. Water evenly instead. Epsom salts have a real use for a diagnosed magnesium deficiency: see the tomato leaf diagnostic.

Slug barriers and traps

Copper tape. The evidence is split. In some studies copper-based barriers have repelled slugs, but the RHS garden trial above found no reduction in damage. Garden Organic recommends copper tape and rings and says it is useful around pots and the legs of greenhouse staging. An unbroken ring on a pot is achievable in a way it is not around an open bed. Keep the ring clean and unbroken, with no leaf or stem bridging it, and do not bother ringing an open bed.

Beer traps. Jars part-filled with beer and sunk into the soil near vulnerable plants are an RHS-suggested trap, to be checked and emptied regularly, preferably every morning. Put them at the edge of a bed rather than among seedlings, because beer draws slugs in, and sink the rim just above soil level so ground beetles, which eat slug eggs, do not fall in; wet weather dilutes and fills an open trap, so empty it more often. That catches slugs, but no evidence shows it reduces overall damage. RHS suggests learning to live with slugs, protecting your most vulnerable plants, and using traps or biological control. Nematodes (Phasmarhabditis hermaphrodita) kill small, soil-dwelling slugs but not snails (Garden Organic).

Baking soda

Potato blight. Washington State University's bibliography of sodium bicarbonate research covers many diseases, including powdery mildews, apple scab and early blight, but has no entry on late blight. That is not proof the claim is false, but there is no research to point to. Potato blight is most common in warm, wet weather and no fungicides are available to gardeners for blight, so do not rely on an untested spray.

Powdery mildew. Washington State University's bibliography lists studies in which bicarbonate solutions controlled powdery mildew on crops such as sweet pepper, applied before infection and sometimes alongside oil, though sodium salts can injure plants and results vary, so treat it as preventative at most. Homemade products are not recommended because they are unregulated and usually untested (RHS). RHS advises providing good drainage but watering regularly during dry periods, as plants under repeated drought stress are more susceptible, and notes that overhead watering in mid-morning in dry weather can reduce powdery mildew, which does not like direct contact with water. See the powdery mildew page.

Cinnamon for damping off

An aqueous cinnamon bark extract at 3 and 5 per cent inhibited the growth of a Fusarium fungus on agar and was described as fungistatic, with further research suggested. That is a laboratory test on one fungus with a concentrated extract, not dry powder on a seed tray against the organisms that cause damping off. Damping off is caused by Pythium, Phytophthora, Rhizoctonia and Fusarium, and no fungicides are available to gardeners. RHS recommends sowing thinly, keeping seedlings well ventilated, using fresh commercial compost, using mains water where possible (or covering water butts), and cleaning greenhouses and water butts regularly. Cinnamon is a low-risk extra, not a fungicide.

Washing-up liquid against aphids

Insecticidal soaps kill small, soft-bodied insects such as aphids by suffocation, disrupting their cell membranes and removing the protective waxes that cover them. But commercial insecticidal soaps are a highly refined version of liquid dish soap, and homemade mixtures carry a substantially increased risk of plant injury. Dry dish detergents and all clothes-washing detergents are far too harsh to use on plants. If you try any soap, spray a small area and wait 24 hours to see if damage occurs, do not spray plants under water stress, and avoid full sun and very hot weather. RHS recommends against using pesticides and suggests squashing aphid colonies between finger and thumb and encouraging natural enemies. See the aphids page.

Common questions

Sources