Thanks for the question.

I suspect that you looked at one of the following sites when you found the recommendation that daylilies should not be planted directly under trees:

https://extension.umn.edu/flowers/daylilies

https://conservancy.umn.edu/bitstream/handle/11299/50495/1/01106.pdf

https://www.gertens.com/blog/growing-and-caring-for-daylilies.html

I believe that this recommendation is NOT because the trees would be affected but rather the daylilies might not get enough water and other nutrients. That you are currently growing daylilies under the linden with no detrimental effects seen either with them or the linden would suggest that further daylily plantings should be OK. However please keep in mind the following:

1). Daylilies thrive in full sun to partial shade. Linden trees typically provide dappled shade, which may be suitable for daylilies if they still receive a few hours of direct sunlight each day. This may not be applicable in your case, since it appears that your current plants seem to be thriving.

2). Linden trees prefer moist, well-drained soil. Daylilies also prefer well-drained soil, but they are more tolerant of dry conditions once established. Make sure the area under the linden drains well to avoid water logging the daylilies' roots.

3). Lindens can be quite competitive for water and nutrients. As I mentioned previously, this was probably the basis for the recommendation on the UMN site. If you add further daylilies under the tree, you may need to provide supplemental water during dry periods and add organic matter to the soil to improve its fertility.

4). I would not add any more daylilies close to the linden trunk. Instead, focus more on the areas closer to the sidewalk.

Good luck. Please get back to us if you have any further questions. Thank you for consulting our forum.