I always use natural light with diffusers, reflectors, and gobos. This requires lightstands, clamps, etc to hold the gear. Articulated arms clamped to a lightstand are very handy.Thus it is not suited for field work. But I work in mine and my neighbors yards so the gear is always handy. It takes time to set up so is a deliberate process. You mentioned that you tried your greenhouse, so this might work for you.
The tulips had two or three large cardboard sheets blocking out the direct sun on the background (gobos). There were a couple of hot spots I could not eliminate and they were fixed in post. The sun was bright and overhead. I used 12-18 inch embroidery hoop with a frosted translucent plastic material (probably shower curtain) held by hand over the flowers. Of course the camera is on a tripod. By moving the diffuser in and out, closer to the flowers, I could judge the light. It was best when the hoop was just above the flowers.
The jonquil is of course back lit. It is very early in the morning so the sun is very low directly behind the flower. I placed a large translucent diffuser, probably a collapsible disk, on a light stand so that the sun was shinning through it for the white background. By carefully adjusting the diffuser up and down I found the position where the diffuser was lit but the sun peaked over the top and back lit the jonquil. It took some juggling to avoid lens flair and line things up.
Both situations were harsh sun and at first glance would not look promising. But with a little effort you can mold the light into something beautiful.
In the field I just look for good light and use small reflectors, mirrors, etc. hand held. Always tripod.