Answer: Established trees can have extensive root systems which will compete with neighboring plants for moisture and nutrients. Trees can also cast a fair amount of shade, providing less than ideal conditions for flowering plants.
In the case of your orange trees, I suspect they are not inhibiting the growth of your sweetpeas by producing acid, but they are interferring by virtue of shallow surface roots robbing your annuals of needed moisture and nutrients.
Try growing an old-fashioned variety that blooms well during summer heat instead of some of the newer kinds that prefer cooler conditions. 'Old Spice Mix' will provide highly fragrant flowers in shades of white, cream, pink, rose, scarlet and purple. Provide full sunshine, rich, moist soil, and enjoy!
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