Answer: Arborvitaes grow best in a sunny location with soil that is rich and evenly moist. The amount of fertilizer to use would depend to some extent on the soil you have in your yard. Your local county extension should be able to help you run some basic soil tests and suggest the appropriate amount of fertilizer based on the result. However, most evergreens planted in average soil thrive on an annual application in late spring of a granular fertilizer such as 10-10-10 per the label instructions as well as some compost spread over the root area. The most important thing you can do is make sure to water if the weather is dry. The soil should be kept evenly moist down through the root area to encourage dep rooting. For this reason an occasional deep soaking is better than a daily light sprinkling. Use your fingers to dig into the soil a little to see if you need to water. Then use a natural mulch several inches thick over the root area but not touching the trunk. This will help the soil stay moister longer. Finally, continue to water if needed up until the ground freezes and again all next growing season. These plants are relatively slow growing by nature as are most evergreens. The larger one will stay markedly larger at least for several years. There is nothing you can do to make the smaller catch up except allow for the passage of time. You could trim back the larger one, but it will regrow more quickly than the other. Unfortunately, if you need a matched pair, it is usually better to purchase identically sized plants. Also, remember to allow for their mature size when planting or else plan to prune them regularly.
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