I planted two sweet autumn clematis. Both have already flowered in July and one has yellowing leaves. Any idea what is happening? Both are south-facing and grew well for a month after planting.
Thank you for contacting the Toronto Master Gardeners about your sweet autumn clematis (Clematis terniflora). Assuming that this is indeed the variety of clematis that you have planted, and not an early summer flowering variety (many of which also bloom again in late summer), then I think that the July flowering is probably the result of a combination of the conditions at the nursery where this plant was grown, and the weather we have had this spring and summer. Higher than normal temperatures and light intensity can help plants to bloom ahead of schedule, as will the use of high-phosphorus fertilizer.
A common cause for yellowing leaves (a condition known as chlorosis) is overwatering. Sweet autumn clematis does best in moist well-drained soil. If your soil is heavy clay or sandy, it would benefit from the addition of compost on the surface of the soil to improve soil structure as it works its way down into the soil over time. Here is an article on overwatering that might be helpful :
Overwatering – Missouri Botanical Garden
Chlorosis could also be caused by nutrient deficiencies, usually iron but also manganese, zinc or nitrogen. Here is a link with more information about chlorosis :
Yellow Leaves Can Indicate Plant Problems
Below is a link to our response to a previous question about yellowing leaves. It also includes a link to an article about overall clematis care.
Best of luck with your clematis !