Good morning,

Growing seasons and winter conditions can affect year-to-year growth, so comparing sizes can be challenging as well. Not to mention the difference in soil, light (sides of yard/house and over-story trees present), etc. on site, even in a small residential yard.

The darker green of the pool-side Thuja behind the slide makes us think of ‘Green Giant’ which Dirr lists as Thuja pilcata while ‘Wintergreen’ is T. occidentalis. ‘Green Giant’ and T. pilcata are darker more lustrous green than T. occidentalis. The panoramic pool photo showing 2014-2019 plantings also shows the apparent difference in greenness; however, it might be sun angles and shading – hard to tell. Dirr says ‘Green Giant’ is a vigorous cultivar and may add 2’ per year. ‘Wintergreen’ is also noted as being a fast growing cultivar, at least for T. occidentalis. ‘Wintergreen’ will be narrower and likely shorter than ‘Green Giant’ at maturity. It is worth noting that ‘Green Giant’ is also reportedly subject to more winter browning/yellowing due to wind desiccation.

You might consider taking cuttings of the different suspect batches and comparing close-up photos of the foliage to determine if there are difference between them. Side-by-side images of cuttings under the same lighting conditions might better show differences in greenness and foliage coarseness that could help distinguish if two different cultivars are present. That approach would be more reliable than comparing growth rates across seasons around the property.