der Busch
bush
A low, woody plant with several stems growing directly out of the ground; a shrub.
Hinter dem Haus wächst ein großer Rosenbusch.
A large rose bush is growing behind the house.

bush
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bush
A low, woody plant with several stems growing directly out of the ground; a shrub.
Hinter dem Haus wächst ein großer Rosenbusch.
A large rose bush is growing behind the house.

The word 'Busch' (bush) is often used as the base word in compound nouns to refer to a specific type of shrub. Examples include 'der Rosenbusch' (rose bush), 'der Holunderbusch' (elderberry bush), or 'der Dornenbusch' (thorn bush).
Im Frühling blüht der Fliederbusch im Garten wunderschön.
In spring, the lilac bush in the garden blooms beautifully.
You can always recognize the idiomatic meaning by the fixed combination with the verb 'klopfen' (to knock). In all other cases, especially when the topic is plants or gardens, the literal meaning 'Strauch' (shrub/bush) is meant.
Er klopfte auf den Busch, um zu sehen, ob sie Interesse hatte.
He sounded her out to see whether she was interested.
The expression 'auf den Busch klopfen' is a fixed unit. It means to ask cautiously in order to obtain information without addressing the topic directly.
Ich bin nicht sicher, ob sie mit dem Vorschlag einverstanden ist. Ich klopfe mal bei ihr auf den Busch.
I'm not sure whether she agrees with the proposal. I'll sound her out.
tree
'der Baum' and 'der Busch' are both woody plants, but they look different. A 'Baum' is usually tall and has a clear trunk that divides into branches higher up. A 'Busch' is lower and often has several branches or stems directly at ground level. The words can be confused because both grow in gardens, parks, or forests. If the plant is large and has a single trunk, you say 'Baum'. If it grows low and densely, you are more likely to say 'Busch'.

Neben dem Weg wächst ein dichter Busch mit roten Beeren.
A dense bush with red berries grows next to the path.
Im Sommer sitzen wir gern unter dem alten Baum im Garten.
In summer, we like to sit under the old tree in the garden.