That's not true, roots will continue to grow in the winter as long as the soil isn't frozen.
I want to echo this. I don't have any charts or studies to present, but my experience watching things grow for 50 years leads me to believe that, yes, there is a quiescent period, some different number of days for each species, but when that period is satisfied, growth returns
when local conditions permit, and they go into and out of a plant ~coma~ when conditions change for the worse. That happens a
lot in SE Michigan over many months from January to April. I see blue Periwinkle flowers in my borders almost every month over winter. Often, in the suburbs where the ground is protected from howling winter by buildings and the sun can warm the land and concrete between houses, and the houses occupy ~20 or 25% of the space, the ground doesn't get much below 30°F. I have measured continually the temperature of strategic parts of my garden for about 20 years (years ago), and 29, 30 or 31°F in the first 6" of soil under mulch, or ground cover, or snow were the most common readings. I used these readings to gauge my gardening activities, for about 20 years.
This, "growing when conditions permit", allows plants to grow in zones ~4 thru 9, etc. The difference in season length can be enormous, but they satisfy their needs for rest and are then ready, willing, and able to grow. There is data for genus Hosta, 700 hours. They are put into dormancy by the progression of conditions of late summer and autumn: completion of ripening seeds, maturing buds for next year, exhaustion of leaf surfaces, reduced heat and sun photo-period/intensity, dryer weather, frost, et al, the precise combination of these cascading factors locally dependent.
How much root growth in winter and early spring? I don't know, I can't see underground, but I can see what I believe is witness to growth.