How to setup django-hstore with an existing app managed by south?

I eventually found that the hstore extension wasn't installed for the specific database I was using:

$ psql -d mydb
psql (9.1.4)
Type "help" for help.

mydb=# SELECT t.oid, typarray FROM pg_type t JOIN pg_namespace ns ON typnamespace = ns.oid WHERE typname = 'hstore';
 oid | typarray 
-----+----------
(0 rows)

mydb=# \dx
                 List of installed extensions
  Name   | Version |   Schema   |         Description          
---------+---------+------------+------------------------------
 plpgsql | 1.0     | pg_catalog | PL/pgSQL procedural language
(1 row)

mydb=# create extension hstore;
WARNING:  => is deprecated as an operator name
DETAIL:  This name may be disallowed altogether in future versions of PostgreSQL.
CREATE EXTENSION
mydb=# \dx
                           List of installed extensions
  Name   | Version |   Schema   |                   Description                    
---------+---------+------------+--------------------------------------------------
 hstore  | 1.0     | public     | data type for storing sets of (key, value) pairs
 plpgsql | 1.0     | pg_catalog | PL/pgSQL procedural language
(2 rows)

mydb=# SELECT t.oid, typarray FROM pg_type t JOIN pg_namespace ns ON typnamespace = ns.oid WHERE typname = 'hstore';
  oid  | typarray 
-------+----------
 58800 |    58805
(1 row)

I thought that a database created after the hstore installation would include the extension. Doesn't seem to be the case, am I misinterpreting how extensions work ? Are they database-specific ?

Django now includes a migration operation to create the hstore extension in PostgreSQL:

from django.contrib.postgres.operations import HStoreExtension

class Migration(migrations.Migration):
    ...

    operations = [
        HStoreExtension(),
        ...
    ]