One periwinkle in flower box size 3” x20”x 6” deep. Full growth with 8” branches. Roots starting to come to surface. Can I trim or have to replant. Not sure what to do


Hello, thank you for contacting the Toronto Master Gardeners for assistance with your periwinkle.  This is a carefree plant known as Madagascar periwinkle or Catharantus roseus.  It is actually a perennial in warmer climates such as Madagascar and Southern Africa where it is native and can become invasive. 

I find it a bit difficult to identify this as a periwinkle as typically the leaves of this plant are a glossy dark green whereas your photo shows the leaves as quite pale, almost yellow.  This is an indication that your plant is under stress.

My first thought was that your window box may be too small, however I think it is large enough to support one plant.  I think instead that you may be overwatering this plant. Periwinkles cannot tolerate soggy conditions.  Does your window box have proper drainage?  Does it have holes at the bottom for water to escape?  This is crucial for proper container plant growth, as the roots of plants unable to get oxygen will quickly rot away.  With the heavy rains we have experienced lately, even with proper drainage your periwinkle likely is suffering from too much water.  

My suggestion to you, once determining that you have holes at the bottom of your planter, is to allow plant to completely drain, and only give water when the top of the soil feels dry.  Meanwhile give your plant a good ‘haircut’ . Take off any leggy branches and if you have not already done so feed with a diluted annual plant fertilizer (the yellowing leaves may also be a sign of nutrient deficiency).  I have included a link that will give you some more information on this plant.  I wish you good luck with your periwinkle.