Question
Hi,
I have a mandevilla dipladenia, crimson sun.  When I bought it, the nursery had some that were trained up quite a large trellis (3-4 foot), but I didn't want to spend that much.  I bought a small one that was 1 1/2 feet tall, very full, but not on a trellis.  I repotted it into a pot with a trellis, but it seems like the vines are not getting any longer, but the plant is doing ok and flowering well.  What do I need to do to get it to grow up the trellis?  Do I need to snip off all the flowers to get the energy to go into growing the vines longer?  Also, I want to overwinter it in my house and I have seen different thoughts on how to do this online.  Some say to put it in a cool dark place and let it go dormant, then trim it back in the spring.  Others say to keep it in a location that gets light and don't let it to go dormant.  Which is right?  I live in zone 6-7.
Thanks for any help you can give me,
Scott

Answer
Scott,
You don't say how long it's been in the new pot, or where you live, so I'll just list a few things that might be going on and you can decide which applies to you.
1. If it's only been in the new pot a couple of weeks, it's too soon to see a great deal of growth up on top. Sometimes plants stretch their roots first, then with that larger root system they are able to put growth on top.
2. If it's been cool and cloudy where you live the plant isn't likely to grow much. I live in the Northeast and it's been a very cool and cloudy June, as I understand it has been in the mid-west as well.  Once the weather is warmer the plant will continue to grow.
3. Keep the plant evenly moist - not wet, and not in standing water, but don't let it dry up as well. If the plant dries up in between waterings the roots will die back and then the plant will be replacing dried roots first instead of growing.
4. Fertilize the plant with a liquid fertilizer once every three weeks - water the plant well before fertilizing - never fertilize a thirsty plant.  Instead of liquid fertilizer you could use time-release such as Osmocote Plus.
5. Since your plant is flowering I'm sure it's getting enough light.


I don't think that snipping off the flowers will alter how fast it grows, but if you want to give it a try there isn't any harm in it.

To overwinter these plants put them in the sunniest window you have - they go through a vining stage in the winter and often will put on a lot of growth although it might be thin and not bloom depending on the light. Water only when the soil is getting dry and don't fertilize. Some people cut their plant down when they bring it in and that's fine too, or you can prune it down in February. Unless you have it under grow lights the plant may not come into bloom until later in the summer - to stimulate flowering earlier, place very close to a grow light and keep that light on for 14 hours a day.

I know some people who keep theirs in the dark all winter and don't water - the plants usually live in a cool basement, and survive, but they don't flower until August.

I hope this helps,
C.L.