So far my Butternut squash plant was doing fine. The first two fruits turned black and fell off after blooming, which I think was because there was no male flower available. Now three more fruits bloomed and I supported pollination by hand and they are growing. However, the next small bumps, which I assume are to be female flowers, are turning yellow and then black, falling off even before blooming. What could be the cause of that ? I'm new to gardening and I'm wondering if I water incorrect or don't offer the right nourishments. The plant is located on a house flat roof in the city (8th floor) and it is my only Butternut squash plant. A Zucchini is right next to it. My neighbor has a Butternut plant too, which is located on the same roof.
There are some dead leaves beneath the still green ones in the center of the plant. Not sure if this is linked.
Edit: The gravel that you see is what the plants are planted in. It is a mix of brick debris, compost, sand and soil that was provided by the building's maintainer. I don't get the impression that it gets very hot during the day. My neighbor's squash is planted in the same substrate and I didn't see their fruits falling off. I used a 2-2-2 NPK fertilizer at first but 12 days ago was the first time that I used a 3-2-5 NP fertilizer (it says tomatato fertilizer that can be used for squash too). I used the amount that is described on the fertilizer for the Zucchini and Butternut plants together since they are very close to each other. But I guess you are suggesting that I need another fertilizer that has more P?
My watering habits are irregular. I sometimes water in the morning, but more often in the evening. I watered a bit less lately because it was raining sometimes but maybe the leaves are diverting the water? I have just been on the roof and removed all the dead leaves and took some more pictures. I saw two new dry leaves, so maybe I didn't water enough?
Also maybe it is worth adding that this happened to some of my zucchinis lately after I successfully harvested two big normal zucchinis already : [ Could be because of no male flower available, but I don't know.