Use a drip system so that the tree is not harmed. Your best investment would be seeds. They can be found in garden shops, all-purpose retail stores, and supermarkets. Catalogs are a particularly good source as they provide information regarding where and how to plant. Seed packages typically list seed count, when and how to sow, sun requirements, and days to harvest.
Packages generally picture seedlings, so that they can be distinguished from weeds.
Collecting seeds and plant divisions from your fellow gardeners is also an effective method of reducing gardening costs. Poor drainage and chilly temperatures may also result in less nitrogen available to the trees. Review your fertilizing and irrigation schedule and check for soil compaction.