The following link contains a short list of branches and whatever to avoid.
Safe branches:
Apple Elm
Ash Magnolia
Beech Maple (except Red)
Cottonwood Poplar
Crabapple Sassafras
Dogwood Willow
Fresh Eucalyptus
Unsafe branches:
Almond Cherry
Apricot Nectarine
Black Walnut Peach
Cedar Pine/Fir
Derudover findes der en hjemmeside som gennemgår et langt større antal at træsorter, blomster mv. som man skal undgå på/ The following link contains a thorough list of branches, flower etc. to avoid: