Hi Yvonne,
community gardens in Germany are organized in the following organizations:

Bundesverband Deutscher Gartenfreunde e.V. (national)
SteinerstraÃe 52
53225 Bonn
Tel. 02 28 - 47 30 36
Fax: 02 28 - 47 63 79
homepage: http://www.kleingarten-bund.de
email: bdg@kleingarten-bund.de

This head organization will also be able to help you finding contacts to Vienna and Prague.

For Berlin and Munich I recommend to try the regional organizations:

Landesverband Berlin der Gartenfreunde e.V.
Spandauer Damm 274
14052 Berlin
Tel. 0 30 - 30 09 32 0
Fax: 0 30 - 30 09 32 69
homepage: http://gartenfreunde-berlin.de
email: info@gartenfreunde-berlin.de

Landesverband bayerischer Kleingärtner e.V.
SteiermarkstraÃe 41
81241 München
Tel. 0 89 - 56 88 83
Fax: 0 89 - 56 76 41

And if you will have some spare time, you should by all means try to visit the new community garden museum in Leipzig (quite near to Berlin). The contact address is:

Förderverein des
Deutschen Kleingartenmuseums in Leipzig e.V.
Aachener StraÃe 7
04109 Leipzig
Tel. / Fax: 03 41 - 2 11 11 94

The museum is opened regurlarly Tuesday, Wednesday, Thursday in between 10 a.m. and 4 p.m., and for guided tours on request also at other times as well as Saturday and Sunday.

Have a nice tour!
Regards, Susanne