Last summer the end of my back-fence trellis that sat behind the south-facing wall stayed green and leafy all season but barely set a flower. The sunnier end of the same trellis, a few feet over, was covered in red trumpet blooms by July. Same plant, same soil, same water. The flowers followed the light. That is the short version of the cardinal climber sun requirements: this vine wants full sun to bloom well.
So yes, plant it in full sun if you want flowers. You can grow the foliage in less light, but the cardinal climber light needs are clear once you watch two stretches of vine side by side. Sun makes blooms. Shade makes leaves. If your goal is a screen of green, partial shade gets you there. If you want the red trumpet color this vine is known for, you have to give it bright, open light.
The reason comes down to energy. Flowers cost a plant a lot to build, and your vine pays for them with the sugar it makes through its leaves in bright light. More direct sun means more fuel, and more fuel means the vine can afford to set heavy clusters of bloom. Drop the light and the plant shifts into a safer mode. It keeps growing leaves and stems, but it holds back on flowers it cannot fund.
That is why a shaded stretch looks healthy yet stays bare of color. Your vine is not sick. It just does not have the spare energy to bloom. Push it into strong light and the same plant pours that energy into the red trumpet flowers that draw in hummingbirds. You see the change within a few weeks of the vine getting better light, since new growth sets buds while old shaded stems stay plain.
Both Clemson and NC State put real numbers on it. Cardinal climber wants at least six hours of direct sun each day for its best flowering, and it only tolerates light partial shade. A little afternoon shade in a hot climate will not ruin it, but deep or all-day shade trades away most of your blooms. Aim for that six-hour mark as your floor, not your goal. The vines I have grown in eight or more hours of sun always carried the most flowers.
Before you commit to a spot, watch it for a full day. Note when the sun first hits the ground and when a wall, eave, or tree starts to block it. You want to count the hours of real, direct light, not the bright open shade that looks sunny but is not. Morning sun followed by afternoon shade can still clear six hours in summer, so a spot that fails in spring may pass once the days grow long.
When you pick a planting spot, go for the sunniest place you have. A south or west exposure with open sky overhead beats a corner that a wall or a tree shades for half the day. This vine earns its keep as a full sun flowering vine, so give it the light it asks for. If your only open ground is shaded, grow the vine in a large pot instead. You can move the pot to chase the sun as the season shifts, and that flexibility often beats a fixed bed in a dim corner.
Trellis placement matters as much as the spot. Set the support so the whole vine catches strong light, not just one face. A trellis flat against a wall puts the back half in shadow, which is what left my own wall-side stems bare. Stand it out from the wall a few inches, or angle it, so sun reaches both sides. A freestanding obelisk or an A-frame in open ground works even better, because light wraps it through the day.
Keep the planting from getting crowded too. If you let other tall plants or a thick hedge shade the base of your vine, the lower stems go bare while only the top blooms. Give the vine its own clear sky and thin out anything that throws shade across it. Do that, and you can watch the red blooms spread across the entire screen instead of clustering on one bright edge. Pick the sun, set the trellis right, and the flowers take care of themselves.
Read the full article: Cardinal Climber: Grow Guide and Care Tips
