In this paper we generalize a result of de Bruijn, Knuth und Rice concerning the average height of planted plane trees withn nodes. First we compute the number of allr-typly rooted planted plane trees (r-trees) withn nodes and height less than or equal tok. Assuming that all planted plane trees withn nodes are equally likely, we show, that in the average a planted plane tree is a 3-tree for largen; for this distribution we compute also the cumulative distribution function and the variance. Finally, we shall derive an exact expression and its asymptotic equivalent for the average height\(\bar h_r \) (n) of anr-tree withn nodes. We obtain for all e>0 $$\bar h_r (n) = \sqrt {\pi n} - \frac{1}{2}(r - 2) + O(1n(n)/n^{1/2 - \varepsilon } ).$$