Yes, turnips full sun exposure of 6-8 hours per day produces the biggest and sweetest roots. They can handle some shade, but giving them the most light you can will always lead to a better harvest. Your roots will be larger and your greens will grow thick and healthy with strong direct sunlight.
Turnip sunlight requirements are a bit more forgiving than most root crops. The USDA NRCS Plant Guide notes that Brassica rapa grows best in full sun but will grow in moderate shade. UF/IFAS recommends a minimum of 8 hours for the best root size. This makes turnips a solid pick for gardens that don't get perfect sun all day long.
I ran a side-by-side test in my own garden two years ago that showed this clearly. One bed got full sun from sunrise to sunset. The other bed sat behind a fence and only got about 4 hours of direct light in the morning. The shaded bed still grew turnips, but the roots came out about half the size of the ones in full sun. The greens, though, grew just as tall and tasted just as good from both spots. That test taught me a lot about what turnips can handle.
The science behind this is simple to grasp. Leaves use sunlight to make sugars through photosynthesis. Those sugars then flow down into the root and make it swell up bigger over time. Less sunlight means less sugar each day. The plant still grows fine, but the root stays smaller since there isn't as much fuel going into it. Greens don't need that same sugar storage. That is why they still do well in spots that get lower light than other parts of the garden.
Growing turnips in shade is doable if you set the right goals for your crop. Focus on greens instead of big roots and you won't feel let down by the results. Varieties like Shogoin and Seven Top were bred for their leaves rather than their roots. These types produce fast and heavy even with only 4-5 hours of sun per day. I grew Shogoin under my apple tree last fall. It gave me three solid harvests of cooking greens from a single planting.
Give your turnips full sun by picking the brightest bed you have when big roots are the goal. Orient your rows running north to south so each plant gets even light during the day. This small change keeps one side of the row from shading the other. Every plant in the line gets its fair share of hours that way.
Tall crops like tomatoes or corn can cast shade over nearby beds as the season goes on. Keep your turnip rows away from these taller plants or plant turnips on the south side where they won't get blocked. I lost a whole row one year because my pole beans grew tall enough to shade the turnips by mid-summer. A bit of planning in spring saves you from that kind of surprise later on.
You can grow good turnips without a perfect sunny spot in your yard. Just match your variety to your light level and set your harvest goals to fit what you have. Full sun gives you the biggest roots and fastest growth by a wide margin. Partial shade of 4-6 hours still works for greens and small roots. Anything less than 4 hours of direct light will leave you with spindly plants that aren't worth the effort or the garden space they take up. Put your best sun toward turnip roots and use the shade for turnip greens.
Read the full article: Growing Turnips: A Step-by-Step Plan
