There's a few ways depending on the plant and severity of the split...
Splice grafting - this allows you to reattach the broken stem to the main plant. It enables the exchange of moisture and nutrients to sustain the damaged stem. You need to determine if the stem is completely severed or still attached by some connective tissue to use this method.
Use a dowel or stake to hold the broken stem upright and bind with plant tape or old pieces of nylon to bind the stem tightly to the healthy material. The binding should have some give to allow the stem to grow. If the stem is dangling, brace it to avoid additional pressure during healing.
Bamboo Skewers are an easy fix too... Measure the broken stem and cut two bamboo skewers to the same length, then place one skewer against the plant’s tender stem and push its end into the soil at least an inch to secure it.
The skewers act as a splint, supporting the broken stem until it heals up. You can then remove the skewers.
Alternatively you can apply florist tape around the broken or bent stem along with a popsicle stick around the stem to provide additional support. Sellotape or electrical tape probably won't work if the stem is thin and soft.
