Lots of people take their left over tea and pour it over the soil of houseplants thinking that it does some good. Tea obviously contains chemicals and they might be good for plants. What about the milk, and sugar? Does it help plants grow?
Are any of the chemicals harmful to plants? Will the mixture cause bacteria or mold to grow in the soil? Sit back, have a cuppa and lets explore this habit of adding tea to houseplants.
What Is in a Cup of Tea?
Tea leaves contain thousands of chemicals. As the leaves are processed, fermented, and steeped into tea, even more chemicals are created and each brand contains different ones. The truth is that much of the chemistry of tea is still unknown. It is estimated that there are some 30,000 different polyphenols, also known as tannins, in tea and that is just one class of compounds. A strong cup can have as much as 240 mg of polyphenols.
Except for a few cases we have no idea what all these chemicals do to plants.
Tea contains about 30 different minerals including fluorine, manganese, arsenic, nickel, selenium, iodine and aluminum.
Pesticides in Tea
It should come as no surprise that tea contains pesticides, after all they are used to grow the plants. CBC Research found multiple pesticides in 8 out of 10 brands, including Tetley, Lipton and Twinings. Half of the samples had levels in excess of allowable limits.
These won’t hurt your houseplants, but will they harm you?
This is another case of, “the dose makes the poison”. Any possible concern really depends on the amounts found in the brewed tea. To better understand this see my article:
New style tea bags are made from polyethylene terephthalate (PET), a plastic, which can also leach chemicals into your brew. PET is a class 1 plastic that is considered safe for food.
The bottom line is that the amount of synthetic pesticides in tea is quite small and does not present a problem unless you consume large amounts. Remember that 99.9% of the pesticides you consume by eating fruit and vegetables are naturally occurring compounds made by plants.
Nutrients in Brewed Tea
In one report, a cup of tea (no milk or sugar) contains 4 to 5% total nitrogen, 2.5% potassium, 0.8% phosphorus, 0.6% calcium and 0.5% magnesium. This report seems to suggest high levels of nutrients, but it is important to look at the details and in this case it is not clear what the numbers mean. The link does talk about “cups of tea” but I suspect the numbers represent % of dry weight of solid content and not the amounts in the brewed liquid tea.
Dry tea leaves contain 4.4% nitrogen, 0.24% phosphorus and 0.25% potassium. The total average level of nitrogen in all plants is about 4%. Clearly you can’t have 4 to 5% in the tea extract, so the above values are clearly not based on the liquid tea.
Another study makes its units clear and reports 0.001% phosphorus, 0.01% potassium, 0.001% magnesium and even less calcium. Nitrogen was not measured.
How does that compare to a common houseplant fertilizer? Schultz all purpose 10-15-10 plant food, is diluted using 0.5 ml/l, giving 0.007% phosphorus and 0.005% potassium. The recommended dilution for Miracle-Gro all purpose 24-8-16 produces 0.008% phosphorus and 0.016% potassium.
Brewed tea provides these nutrients in roughly the same amounts as fertilizer. Phosphorus is a bit on the low side.
Milk and Sugar With Your Tea
Both milk and sugar contain organic chemicals that help feed microbes. You might not want these growing in houseplants, but the reality is that they are there and they don’t normally harm the home environment or your plants.
They will both provide slow release nutrients as the microbes grow, digest the organic matter and die. Milk might stink a bit as it ages, but I suspect that is a minor problem, if it exists at all.
pH of Tea
- Black 5 to 5.5
- Green 7 to 10
- Herb 6 to 7
Small amounts of tea are not likely to affect the soil pH, but if tea is added daily, the pH of the soil solution will start being very similar to the pH of the tea.
If you have hard water with a higher pH you might want to stay away from green tea, but the low pH of black tea is not good for you teeth.
Growing Seeds in Used Tea Bags
A bit of a segue, but I found this suggestion while researching the above. Take an old tea bag, make a hole, and plant a seed.
What a stupid idea!
Nitrogen Infused Tea
This is a new fad where nitrogen gas is added to regular tea to create a fuller textured brew. Since the nitrogen is in the form of atmospheric gas which plants can’t use, this should have no effect on plants.
Should Tea be Poured on Houseplants?
I would not brew tea to use as a fertilizer for houseplants. I doubt it is cost effective and brewing the water wastes hydro. But using old tea that will not be drunk, will provide some nutrients for plants, and provided it is not done in excess, should not affect the pH of the soil or cause any other harm to the plant. It is probably better than just dumping it down the sink.