The leaves swell up and push out of the infuser

In short

Dry leaf expands to several times its volume, and rolled and compressed teas expand most of all. Overflow is a sign that the leaf was measured by eye against the container rather than by weight against the water.

Likely causes, in order

Most likely first. Answer the question under each one before moving on — several of these look alike in the cup and are told apart only by what was done to the leaf.

1. 1. Filling the basket by volume rather than weighing

   Leaf quantity

   Check: Did you fill the basket to look right, rather than weighing 2 to 3 g per 200 ml?

   Fix: Weigh it once. A basket that looks under-filled with dry leaf is usually correct; one that looks properly full is usually two or three times the dose.

2. 2. A rolled or compressed tea, which expands most

   Leaf quality

   Check: Is it ball-rolled oolong, gunpowder, or leaf broken from a cake?

   Fix: These expand several times over. A vessel a third full of dry pellets will be entirely full of wet leaf, which is the standard warning and is routinely ignored.

3. 3. The basket is too small for any sensible dose

   Equipment

   Check: Is the basket a small ball or a narrow cylinder?

   Fix: Replace it with the largest basket the mug will take, or abandon the basket entirely and use a strainer on the way out. A cramped basket under-extracts even when it does not overflow.

4. 4. Filling a paper filter bag more than half full

   Equipment

   Check: Does the bag look comfortably full before brewing?

   Fix: Fill filter bags no more than half. A packed bag brews unevenly and splits at the seam when the leaf swells.

Tea types this applies to

• Oolong
• Dark tea and pu-erh

Brewing methods this applies to

• Mug infuser brewing — Loose leaf in a basket inside a mug — the most common way loose tea is actually made, and the method most often responsible for people concluding that loose leaf is not worth the trouble.
• Tea bag brewing — Pre-portioned tea in a permeable bag. The format was an accident of early twentieth-century sampling practice and became the dominant way tea is drunk worldwide, because it removes every decision except how long to leave it in.

Related problems

Sharing a cause category, a tea type or a brewing method with this one.

• My clay pot has changed how the tea tastes — A tea that tasted right in porcelain comes out muted, differently scented, or carrying something that was not in the leaf.
• My first infusion is always the worst one — The opening cup is thin, dusty or flat, and the second is markedly better — consistently, across different teas.
• My gongfu tea is bitter even at five seconds — The timer says five or ten seconds and the cup is over-extracted anyway, with the later infusions dropping off sharply.
• My tea is harsh and I cannot tell whether it is bitter or astringent — The cup is unpleasant in a way that is hard to name, and every piece of advice about it assumes you already know which of the two words applies.
• My tea tastes of chlorine, metal or the kettle — A chemical, swimming-pool or metallic note sits over the tea, and it is there whichever tea is brewed.

Sources