tomatoes fail to set fruit when the heat of summer sets in and also if they are in too much shade or overfertilized with nitrogen. Yellowing leaves is either due to a soil moisture problem (too much or not enough) or to a leaf fungus. Look for spots on the foliage and if you see a lot especially on the yellowing foliage spray with a fungicide labeled for tomatoes such as something containing maneb, mancozeb, benomyl or copper based sprays.
The best time to plant roses is in fall (Oct & Nov). Winter and early spring are also good. You can plant container grown trees and shrubs at any time of the year but in the hot summer months you'll just have to take a lot more care to keep the root ball moist without overwatering which is deadly.
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