Sorry I'm not sure where would be best to post this message - are plants from Madagascar considered tropicals? When I bought a couple of these plants as a few small, spiky twigs I wasn't sure how they were going to do, but they have really been taking off! They are now over three feet tall and about as wide as well, and I'm wondering how/when I should prune them to keep their growth in check. The way they are growing now, I feel like I could just hack them to the ground and they would be back to this size by winter, but I don't think that's the way to go since I have basically no knowledge of how Solanum grows. What is the best time of year to prune these plants? And how much should I take off? Thanks for any info!
Welcome to Tropical Plants! Yes, anything from Madagascar would be tropical, I think.
Hm, since nobody's answered you yet, I'll give it a guess. To be honest, I've never seen this plant before, and I went to the Plant Files where usually you can find everything, but it's not there either.
First, you probably know anything with 'Solanum' in the botanical name is in the nightshade family, same as tomatoes, potatoes and a host of other interesting plants. Drawback here is a lot of them are quite poisonous, so I'd be on the lookout for your plant to make seeds or some kind of fruit, and assume it's dangerous if you have pets or small children that might get too interested. (although as prickly as it is, maybe it's fairly safe?)
If it were my plant, I'd first get a pair of those gardening gloves with the long gauntlets up to your elbows - those are some impressive thorns on that puppy! Then, as the flowers start to fade, prune a couple of the longest stems back by half and see if it branches and keeps blooming. IF it does, prune the rest of the plant accordingly.
I'm assuming you're growing it for the flowers, yes? So if you want to keep the flowers coming, pruning back and not letting it set any seed pods is the way to go. After pruning, a light dose of fertilizer will help it jump up with new growth and bloom again.
We have a local native Solanum here which is also covered in thorns, S. echinatum. Echinatum means spiny. It's highly toxic, unlike some Solanums that are edible. We have 43 species listed for the region and a few are even used commercially for food. Unknown Solanums are not worth taking a risk with to see if they're edible.
I've not known anyone who grows them here. But if you let some seeds go to get some back up plants then you can experiment a bit with different ways of pruning.
Madagascar for the most part is tropical but there are highland areas where you'd get plants that require colder conditions. So it would depend on where in the country a plant came from.
I have had this plant off and on for about 10 years. It will take severe cutting back during the growing periods (spring through early fall) but tends to die back to the hard wood during the winters in LA. It is tropical in that it can't take real cold temperatures, as I mentioned frost will kill it back to the hard wood. However, it is somewhat drought tolerant once established. It will self sow if you allow it to fruit and leave the pods on the plant. It has always come back for me in the spring no matter how sad it looked during the winter. I would recommend cutting it back as it can get to be a large bush otherwise. http://davesgarden.com/guides/pf/go/55483/