Is tea good for plants?

In the UK, the dregs from the teapot were traditionally poured onto plants – often a hydrangea by the front door. With the rise of the teabag and the demise of brewing loose leaf tea in a pot, the habit of sharing tea with plants has largely died out. But is it something we should think of doing again?

From a plant (and human) nutrition viewpoint, the principal agents of interest in brewed black tea (Camellia sinensis, aka PG, Tetley, Typhoo, etc) are the minerals. And of the minerals, tea is particularly rich in potassium and manganese, with appreciable fluoride levels too.

Potassium is familiar to growers – it is the K in NPK (nitrogen, phosphorous, potassium). The reason it has the chemical letter K is because it used to be known as kalium.  Incidentally, in fertilizers it is generally in the form K2O – potassium oxide. For plants, potassium is of tremendous importance. It is classed as an essential macro-nutrient. Essential,because without it the plant will not thrive and macro because it is needed in relatively large quantities. Growers associate potassium with fruits and flowers – that is why, for example,
tomato plant food is high in potassium. However, it is fundamentally involved in many processes within the plant all year round. Deficiency of potassium makes the plant generally more susceptible to damage from pests and from the environment (frosts, drought, etc).

The amount of potassium in the average cup of tea will depend on many things, including where the tea plants were grown, how they were processed, and what the mineral content of the water used to make the cup of tea was. The USDA gives a value of around about 80mg of potassium in an 8oz (237mg) cup of black tea made with tap water. This is about the same amount of potassium as is in a couple of ripe cherry tomatoes.

Interestingly, tea drinking is a major source of potassium for many people. The recommended daily intake for potassium is 3500mg for an adult in the UK, or 4700mg if you’re an American… I guess that’s just the vagaries of national science policies, not some biological difference between the two populations.

Manganese (Mn) is less well known (and not to be confused with Magnesium – chemical symbol Mg).  It is also an essential component in plant nutrition, but is classed as a micronutrient as it is only required in very small amounts. Manganese plays a crucial role in photosynthesis and also in many other processes within a plant. Its role is complex and it often works in conjunction with, or sometimes instead of, other minerals. Nevertheless, it is essential.

As with potassium, the manganese content of your average cuppa will vary widely. However, the USDA provides a value of 0.5mg per 8oz cup of black tea made with tap water.

There is no established figure for the recommended daily amount or intake of manganese in human diets, therefore guideline figures are provided instead. The guideline amount for adults is in the range of 2mg – 5mg per day. In the US, intakes of more than 11mg are considered to be “possibly unsafe”. Again, in human nutrition, tea often plays a major role in providing manganese.

Tea is also somewhat acidic – again, depending on many other factors, particularly the water used to make the brew and the length of time the tea steeps. This probably explains the tradition for “feeding” tea to acid loving plants.

On balance, sharing your cup of tea (once it’s cooled, obviously) with your plants and garden is unlikely to do any harm and is quite likely to be beneficial, particularly to plants that have a high demand for potassium (e.g. tomatoes, strawberries, fruits in general) and those which prefer an acid (low pH) soil.

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