Olá, Tarso e Emílio.
Tentei usar bend left ou bend right, mas, não deu muito certo. Buscando mais um pouco na internet achei a função arc.
\draw[ultra thick, ->,line width=1.5pt, -stealth, red!80!blue] (-11.5,-9.5) arc (-90:180:0.8);
Mas dá um trabalhão pra achar a posição certinha.
Sobre aquela árvore que você me passou, eu não consegui modificá-la. Eu precisava que ficasse um nó na esquerda e outro na direita.
Acabei usando a técnica que o emílio passou, o código acabou ficando assim:
\begin{tikzpicture}[node distance=3cm, scale=0.5, transform shape]
\node (21) [connector, minimum height=1cm, text centered, text width=1cm, below right of=raiz, xshift=4cm] {21};
\node (22) [connector, minimum height=1cm, text centered, text width=1cm,below right of=21] {22};
\node (19) [connector, minimum height=1cm, text centered, text width=1cm,below left of=21] {19};
%-----------------------------%
\node (20) [connector, minimum height=1cm, text centered, text width=1cm,below right of=19] {20};
\node (18) [connector, minimum height=1cm, text centered, text width=1cm,below left of=19] {18};
%---------------------------------%
\node (8) [connector, minimum height=1cm, text centered, text width=1cm,below left of=raiz,xshift=-4cm] {8};
\node (10) [connector, minimum height=1cm, text centered, text width=1cm,below right of=8] {10};
\node (7) [connector, minimum height=1cm, text centered, text width=1cm,below left of=8,xshift=-0.5cm] {7};
%----------------------------------%
\node (11) [connector, minimum height=1cm, text centered, text width=1cm,below right of=10] {11};
\node (9) [connector, minimum height=1cm, text centered, text width=1cm,below left of=10] {9};
%---------------------------------%
\node (5) [connector, minimum height=1cm, text centered, text width=1cm,below left of=7,xshift=-0.5cm] {5};
\node (3) [connector, minimum height=1cm, text centered, text width=1cm,below left of=5,xshift=-0.5cm] {3};
\node (6) [connector, minimum height=1cm, text centered, text width=1cm,below right of=5] {6};
\node (4) [connector, minimum height=1cm, text centered, text width=1cm,below left of=6,xshift=0.5cm] {4};
%----------------------------------%
%\draw[ultra thick, ->,line width=1.5pt, -stealth, red!80!blue] (0.5,0) arc (270:0:1.5);
\draw[ultra thick, ->,line width=1.5pt, -stealth, red!80!blue] (-11.5,-9.5) arc (-90:180:0.8);
%----------------------------------%
\draw [arrow] (raiz) -- (21);
\draw [arrow] (21) -- (22);
\draw [arrow] (21) -- (19);
\draw [arrow] (19) -- (20);
\draw [arrow] (19) -- (18);