I guess you are right.
those definitions I have found:
Muttergarten (3)
Muttergarten , ein Garten, oder eine große Anlage, wo ...
... Sickler zu Klein=Fahnern in Thüringen, zu Anlegung eines Muttergartens, um eine ganze Provinz stets mit guten und ...
Muttergartens zur Verbesserung der Obstkultur so vieler tausend Landes=Inwohner ...
Muttergarten
Anpflanzung von Unterlagsreben, welche Holz zur Veredlung liefern sollen
Gefunden auf http://www.enzyklo.de/Begriff/Muttergart

http://www.kruenitz1.uni-trier.de/xxx/m/km11053.htm

Deutsch term (edited): Unterlagen-Muttergarten

Compared to gardens grown on their own roots which can start yielding in one-and-a-half year, therootstock gardens take 6-8 months more since they have to be first planted in the field and then grafted with suitable scion variety after 6 months.
http://www.icar.org.in/files/ar0304/04-HORTICULTURE.pdf


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Note added at 6 hrs (2012-02-08 04:37:35 GMT)
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IOW, the source term Unterlagen-Muttergarden needs to be translated in its entirety.

A very common translation for "Mutterpflanze" is also "parent plant". That is the reason why I would suggest Parent Plant nursery which implies all plants that serve as ancestors for the following generations.