It depends how complicated you want to get, what sort of range you need, and how large
the plants are. (We have hydrangeas that are large enough to insert the tracker into a
hole in the trunk. But more likely you are dealing with much smaller plants if the neighbors
are making off with them.)
A deluxe solution might be to use one of the APRS trackers sold for balloon launches and such -
they include a GPS unit to send their current position, and if there are within range of a digipeater
you can track the plant across the country. Like this:
http://www.argentdata.com/products/aprs.htmlAnother option would be an "FM Wireless Mic" operating in the FM broadcast band. Here
is an example you can build:
http://www.sm0vpo.com/tx/bug.htmYou can build such a device for 2m just as easily, and adding crystal control isn't too difficult.
However, a simple crystal oscillator is probably easier (without the microphone circuit). The
hard part is finding suitable crystals, but baud rate crystals that multiply up to 147.456 MHz
(fundamentals of 12.288 MHz, 8.192 MHz, 16.384 MHz, etc.) are stock items in most
electronic catalogs. A 16 MHz crystal oscillating on the 9th overtone will hit the 2m band
with just one stage. Such a stage can be powered by a 9V battery, or a small hearing-aid
size cell if one wants to get quite small.
To allow the battery to last longer, add a CMOS 555 timer chip that pulses the signal at a
low duty cycle. This does make it more difficult to hunt, but can extend the useful life by
10X or more. You'll have to see how long it takes your radio to register a signal and display
the signal strength, and make sure that your transmit pulses are at least that long (or look
for a local ham with better equipment that will help you.)
There are also some 433 MHz transmit chips available that would make it easier to build
something for that band - the higher the frequency, the smaller the antenna for the
same gain and/or directivity.
Suitable antennas can be built quite cheaply using WA5VJB's "Cheap Yagi" methods here:
http://www.wa5vjb.com/yagi-pdf/cheapyagi.pdf