Yes, full sun annuals partial shade will grow but you get fewer flowers and weaker plants. Most sun-loving annuals can survive on 4-6 hours of direct light. They just won't put on the show you hoped for in your garden beds.
I learned this the hard way with my zinnias a few summers back. I planted them in a bed that got morning sun but afternoon shade from a big oak tree. The plants grew tall and leggy with long gaps between leaves. They made about half the flowers my other zinnias did in full sun. My marigolds in the same spot did even worse.
The sun requirements for flowers come down to basic plant science. Your annuals use sunlight to make food through their leaves. Less light means less food for the plant to work with. When food runs short, flowers are the first thing to go. The plant puts its energy into staying alive instead of making blooms.
Mississippi State Extension notes that most annuals from semi-arid places need full sun to thrive. These plants evolved in open spaces with little shade at all. They just don't have the tools to do well in darker spots in your yard. Their leaves are built to handle bright light, not work well in dim conditions.
Annuals in shade also tend to grow weak and stretched out. They reach toward whatever light they can find. This makes them floppy and prone to falling over in rain or wind. The stems never get thick and strong like they would in full sun. I staked my shady zinnias three times that summer. They still looked sad and lanky next to their sunny cousins.
Some full sun annuals handle partial shade better than others in your garden. Vinca does fairly well with 4-5 hours of sun and keeps blooming. Impatiens is listed as a shade plant but many people don't know it can take part sun too. Wax begonias split the difference between sun and shade without much trouble.
You can help your annuals do better in less than perfect light. Plant them where they get morning sun rather than afternoon sun. Morning light is softer and gives plants a good start to the day. Feed them a bit less than you would in full sun spots. Extra food pushes weak growth when light can't keep up with it.
Watch your plants through the first month after planting them. If they stretch toward the light and get leggy fast, the spot is too dark. If they stay compact with good color, you found a winner. Move any strugglers to sunnier spots before they waste the whole growing season. Your efforts pay off when you match the right plant to the right amount of light.
Read the full article: Full Sun Annuals That Thrive in Sunshine
