It's always in the summertime that we get inundated in calls at the Helpline about dying redwoods. I was just going through my archives and found a great answer to an incoming email about dying redwoods. Because water is such a valuable resource, I won't just hint around about it, but redwoods are native to areas of California that have about 40 inches of rainfall per year, mild weather and heavy amounts of fog. Average rainfall in Paso Robles is about 15 inches, low humidity and often has very sandy/silt soil that drains quickly.
Here's the question:
We are still trying to find a solution to our dying redwood trees. The irrigation schedule is now 2 gals per tree, per day, 4 days a week. The trees were shipped in 15 gallon plastic containers. The trees were planted with sufficient room for root growth and to allow for the planting mix to be placed at the base of the hole, as well as around the tree. The planting mix was approx. 20% compost, 80% natural soil. We have had some hot weather lately. What effect can this heat have on these trees? I appreciate your feedback.
There are procedures for determining the water needs of plants in a landscape. Your particular situation on the edge of the Nipomo Mesa in a coastal area means that exposure is going to bring up the water needs of the plants. Additionally, redwoods are high water demand plants and obtain much of their needed water from fog drip. Somewhat more limited in your exposure.
We can estimate these conditions numerically, and what those calculations indicate is that your redwood trees will use about 3/4 of the water that a lawn planted on the same total area would use. Now we do have good data on the water needs of a clipped grass lawn in these coastal areas, and the requirements are about 4.6 inches of water during July. So your trees are need about 3.5 inches of water. Think of it like a rain gauge but upside down. That's a tough concept to convert to how long to run your drip system.
You can calculate the area for each tree, and we know that it takes a little over 27,000 gallons of water to cover an area 209' by 209' (an acre). If you convert that 3.5 inches of water, corrected for the efficiency of the irrigation system and for the spacing of your trees, that means each tree is using about 10 gallons of water per day. Yes, per day.
Your weekly irrigation amounts are 8 gallons per tree per week, but the tree needs 70 gallons per week. For a period of time, the tree will be able to use water that is stored in the soil from winter rainfall. We know that in most sandy soils, there is about 1 gallon of water per cubic foot of soil. The later the spring rains and the greater the number of foggy days, the less water you need to apply with the drip system. We did have some later spring rains, but it has been clear and hot most of June and July.
Redwoods benefit from less frequent deeper water than will be supplied with short irrigations. So having said all of this, here is my recommendation.
1) Identify 2 trees - one that is doing well, and one that is just starting to look like it's suffering. Don't pick a 'dying' tree because it's likely already gone.
2) Run your irrigation system for the normal two hour set. Wait until the next day before going to step 3.
3) Start about 2 foot from the edge of the canopy of both trees and dig a trench towards the trunk that goes down at least 2 feet if not more. Look for moist soil and roots. If there is adequate soil moisture, you will be able to form a ball with the soil when you hold it in your hand. If the soil moisture is low, then you won't be able to form a ball. You should find roots from the tree throughout the trench. Fill in the trenches.
4) Next, run your irrigation system for 6 hours and repeat the above on the other sides of the same trees on the day following the irrigation. Look to see how much more water movement you've gotten from this longer irrigation. This irrigation would be supplying enough water for the tree for one day.
Let me know what you find, and if you can send pictures that'd be great!