Kozjanski Park is an extremely valuable area when it comes to the heritage of exceptional trees. At least 12 respectable trees are older than 250 years. Some of them have never been documented.

An oak called Nujčev hrast in Gregovce, the largest of its kind in Slovenia, deserves special attention and so does a linden called Bizeljska lipa. 15 Ilex aquifolium trees are over 10 metres tall, and this species spreads by itself in the area of Mrčna sela, which is rather unusual for this Part of Slovenia. There are also some linden trees that are older than 200 years and their trunk circumference is over 400 centimetres. You can also admire an exceptionally old and large Sorbus domestica, one of the largest yew trees in Slovenia (yews are successfully spreading by themselves in Tisovec reserve), some chestnut trees of amazing dimensions, unusual firs, sequoias, Gleditsia sp., Cryptomeria japonica, planes, Catalpa bignonioides in the Pišece Castle and many others.