Avocados in June - 1992
No matter where they grow in California, June is a month when avocados are being watered on a regular schedule. How regular that schedule is should be carefully reviewed by the irrigator. In 1991-'92, at our Thornhill/Camarillo irrigation plot we applied 32" of water, but in '92-'93 we put on only 26". Same trees, nearly the same size, but a 23% difference in applied amounts dictated by differences in water demand due to different weather. The irrigation schedule we use is driven by tensiometers and a CIMIS weather station. The station generates reference evapotranspiration values which tell us how much water to apply at an irrigation, and the tensiometers are used to verify whether the trees are doing well by the schedule. Irrigation on a fixed schedule, such as once a week for 24 hours, is going to guarantee that on average you will be either under or over irrigating at each irrigation. Using some soil-based measure, such as a soil probe or tensiometer can assure an irrigator that trees are getting the appropriate amount of water when they need it.
In orchards which have not closed canopy yet, weeding is an ongoing activity. In a research plot, we are using tensiometers to monitor the effects of weeds, bare soil and chipped yard waste mulch around trees. A typical soil moisture profile shows 80 cbars of tension at 6" and 60 cbars at 18" of soil depth in the weedy plot, 60 and 30 at 6" and 18" in the bare plot around the trees, and 40 and 30 in the mulched plot. Centibars is a measure of moisture tension, the higher the value, the drier the soil. As trees get older they make their own leaf mulch and shade which limit weed growth. There is no question that a cover crop can improve soil conditions through reduced erosion, improved water infiltration, and possible reduced disease and pest problems. These soil improvements tend to improve tree growth and orchard productivity. But, if water is the primary issue, weeds and a cover crop can add considerably to water use in an orchard, especially a young one. Weed control through the use of mulches and herbicides can effectively reduce the water requirements of trees.
June is still a good time to replant an orchard. The soils are warm enough to give the trees a good start and there is enough fine weather left for them to establish before winter comes. Late plantings (September, October) are discouraged because the root shocked plant sits in a cold, wet soil through the winter and becomes a prime candidate for root diseases. Especially in a replant situation, it is a good idea to start them off with a fungicide, such as Ridomil¨ or Aliette¨, to give them some protection until they get established.
At the time of writing, Aliette¨ has still not received its full registration, but it is supposed to be around the corner. The label will be for foliar and irrigation injection only, not tree injection which has been shown to be detrimental to trees in the long run. The best time for Aliette¨ to do its job is when there is a good root flush of growth which occurs after the leaf flush in spring and fall.
When replanting, try as much as possible to avoid interplanting between older trees. The different water requirements of the young and old trees is such that one or both will be stressed because they need different schedules - less but more frequent for the young trees. Attempts are made to put together a system where the older trees remain on the 10 or 15 gpm sprinkler and the young trees are put on a 1 gpm dripper, but this still can not be an ideal situation. The best thing to do is to clear out trees with in an irrigation block and replant, or replumb a block with a new valve so that small new block can be irrigated differently from the older trees. Where clonal rootstocks fail in a root rot replant situation is invariably where water control is lacking or poor.
And don't forget, it's never a good time to thin trees that are overgrown, but once the trees are picked this year, resolve to begin that thinning program that has been delayed these many years. It's good for you and the trees in the long run.