There was a 2011 study where they used a sensitive atomic magnetometer to try to detect a plant's magnetic field. They stated that:
To our knowledge, no one has yet detected the magnetic
field from a plant. Biochemical processes, in the form of ionic
flows and time varying ionic distributions, generate electrical
currents and time-varying electric fields, both of which produce
a magnetic field. However, contrasted to muscle contraction
and brain processes, which have a characteristic time
scale shorter than one second, plant bioprocesses span several
minutes to several days and the expected magnetic field from
such processes is correspondingly smaller.
Measurements with a sensitive atomic magnetometer were performed on the Titan arum
(Amorphophallus titanum) inflorescence, known for its fast biochemical processes while blooming.
We find that the magnetic field from these processes, projected along the Earth’s magnetic field,
and measured at the surface of the plant, is <
So according to this, no, you wouldn't be able to sense the magnetic field from a plant with a magnet.