Angular Material 6.0.1 Tree default opened and expand/collapse all

In addition make sure that treeControl nodes are set. After loading data you should

treeControl.dataNodes = nodes; 
treeControl.expandAll()

See this github issue

MatTree's treeControl provide a expandAll method which you can use to expand all tree nodes, and collapseAll to close all tree nodes.

You can can instance of MatTree via ViewChild and call expandAll in ngAfterViewInit life hook to make it expand by default.

@ViewChild('tree') tree;

ngAfterViewInit() {
  this.tree.treeControl.expandAll();
}

Source example for calling from template:

<button (click)="tree.treeControl.collapseAll()">collapseAll</button>
<button (click)="tree.treeControl.expandAll()">expandAll</button>
<mat-tree #tree [dataSource]="dataSource" [treeControl]="treeControl">
  ...
<mat-tree>

see example.