Piperazine-2,3-diones as plant growth regulators

Piperazine-2,3-diones of the formula ##STR7## where R.sup.1 and R.sup.2 are identical or different and each is C.sub.1 -C.sub.20 -alkyl, C.sub.3 -C.sub.18 -alkenyl or C.sub.3 -C.sub.8 -alkynyl, where these groups may carry from one to five halogen atoms;

• C.sub.3 -C.sub.10 -cycloalkyl, C.sub.5 -C.sub.10 -cycloalkenyl, C.sub.3 -C.sub.10 -cycloalkylmethyl or C.sub.1 -C.sub.4 -cycloalkenylmethyl, where the rings may carry from one to three C.sub.1 -C.sub.4 -alkyl groups or a phenyl ring or norbonyl, adamantyl or tricyclodecamyl;
• or phenyl, phenyl-C.sub.1 -C.sub.4 -alkyl or phenyl-C.sub.2 -C.sub.6 -alkenyl, where the aromatic radicals may carry from one to five halogen atoms and/or from one to three of the following groups;
• nitro, C.sub.1 -C.sub.4 -alkyl, C.sub.1 -C.sub.4 -haloalkyl, C.sub.1 -C.sub.4 -alkoxy, C.sub.1 -C.sub.4 -haloalkoxy, C.sub.1 -C.sub.4 -alkylthio or C.sub.1 -C.sub.4 -haloalkylthio.