I over-wintered 2 mandevillas and both plants looked awesome during the summer and just in the past couple of weeks I noticed each had 2 flowers. Do I over-winter them again or let them die?
Over-wintered mandevilla not blooming
Definitely over winter them! Start feeding them in May and you should have bountiful blooms next year.
I had two of them for 18 years before they finally got mealy bug, likely from plants near them on our indoor, unheated porch. Now I have two new ones. At my age that's a lifetime supply!
Pirl what do I feed them? Miracle Grow? Have you ever taken cuttings?
Miracle Gro is fine but cease feeding by September. You don't want new growth just to have to cut it back hard when you bring it inside. I have never tried cuttings of it since I wouldn't know where to plant more of them if the cuttings worked.
To take 3 / 4 inch cuttings, of lateral shoots or with pieces of woody stem if no new soft shoots have grown, you need do this in March / April and use new shoots from the early spring growth, if you use cuttings with woody stems you do this in June / July, you need well drained but rich soil so I would buy good potting mix from the garden center and add a handful of (horticultural sand) or some small grit to the soil as this will give the good drainage the plants need, bought compost is normally sterilized therefore wont spread decease like old compost or garden soil for cuttings as this plant can be fussy about soil when starting off.
Insert the cuttings into the compost and place the shoots around the edge of the pot (you can set 4/6 cuttings into the edge of a pot easy, this will allow for any loss of cuttings. place the pot of cuttings in a saucer of water till the soil turns a darker colour indicating the soil has gone wet enough and then drain the excess water away, don't let the cuttings or mature plant sit in water at the bottom or the stems will rot before roots grow.
Once the cuttings have rooted, re-pot them into next size pot but, individually, keep indoors in a warm area but NOT in direct sun, just a bright area,
As with new plants or mature ones, these plants like a humid atmosphere while growing so I mist mine every other day with tepid water, Keep a bottle of water indoors close to plant as this water will be at the same temp as the compost in the pot. only water the compost when it feels dry (finger test) give a drink and leave till you know compost is drying out or you see the nice lush shinny leaved wilt.
As for why your plant is not flowering, it could be too much direct sun on the leaves, not enough light, soil too wet, not letting the plant rest over winter, You stop watering about end of September / October and just give enough water to stop plant dying off, gradually begin watering again in spring, carry on watering till early autumn when the flowers stop and then rest winter again.
For feeding I use a seaweed liquid extract but any liquid feed for indoor plants will be fine, start of half strength for a few weeks and when you see more growth, increase to full dose, always read the bottle for how much as over feeding is just as bad as no feed.
when the plant grows tall and lots of branches grow, make a climbing frame for the pot so you can weave the new growth in and around the frame, to encourage lots of new branching, I nip out the growing tips of the branches early spring, this helps make the plant make even more side shoots, remove dead or wilting flowers as the are seen to keep new flowers growing.
Remember this plant has a white milky sap so wash your hands after nipping /cutting etc, it may cause itchy skin.
Don't give up on your plant as mine took 2 years to get going, I think also I had it too cold in winter for it and maybe even too wet but have resolved these problems and my plant flowers right into December some years (indoors) and I have to make myself give it a rest, but as I said, the plants are slow to get going at first, I think they like to make more roots than top growth first year or so.
Hope all this is a help and you get your plants to grow like Topsy, they are beautiful red, pink or white flowers and lovely shinny leaves too, good luck. WeeNel.