What material and current do I need for a diy plant heater?
The heating element should be electrically isolated from the soil.

You can buy resistive heating cable designed for in-ground use. 

You can buy cable intended for heating floor slabs. 

For small one-off playings a water-bed heater may be useful (if you can still find one. LOW VOLTAGE versions are essential if you wish to live long and prosper. 

You could make your own resistive cables using eg Nichrome wire in a piece of hose. You would need to be sure that maximum temperature rise did not damage the hose.

Many cables mentioned on web. eg

TYCO - very reputable. Available in MANY countries.

Nexans "a global group"

Eltherm -  a range of related products

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