Answer: Begonias are usually quite trouble free, and I would not expect any problems. Check them periodically in case you ever do see something unusual but I would not expect it. There is no "preventive" spraying you would do for these plants.
When you water it is more efficient to apply the water to the soil at the root area and not over top of the plants, the best way to do this is probably with a soaker hose if you have a lot of plants or water them with a watering can or tricklinghose one by one if you have just a few.
However, you probably will not need to water these much, if at all. You water to supplement rain, if needed. This means you may not need to water much until July and August if normal rain keeps the soil damp. Your goal in watering is to keep the soil slightly moist, not saturated or sopping wet. Water them well at planting to settle the soil, then apply a layer of organic mulch over the soil. It should be several inches thick. This will help keep down weeds and also help maintain soil moisture.
To know if you need ot water, pull aside the mulch and dig into the soil with your finger. If it is still moist you do not need to water yet. When you do water, water slowly so it soaks into the soil. You want to water thoroughly to encourage deep roots. It is better ot water deeply less often than to sprinkle lightly every day. After you water, wait a few hours and then dig down into the soil to see how far the water went; sometimes it can be surprising. For annuals such as these, you want to water so that it is damp about six inches down.
Enjoy your begonias!
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