How to set up perennials under trees?

Any suggestions on how to tweak the advanced settings for this situation? We have perennials and shrubs under some well established (tall) trees. The trees of course take a lot of water out of the soil. But I don’t need to worry about watering the trees (after all, they were here for a hundred years without irrigation), only the perennials and shrubs.

If I configure it as if it were a perennial or shrub zone, the soil is constantly dry. What factors are the best to adjust to get adequate water to those plants?

Thanks for asking this, @stonecliff. I have a very similar question, and I’m betting the logic to the right answer will be the same. I have drip zones with a mix of perennials and annuals. Most of the perennials do pretty well no matter what I’ve set, but the hydrangeas seem to want a ton of water when it’s hot to keep them from wilting, as do the annuals. Last year I had the zones set up as annuals, but I don’t know if I was watering deep enough. And to complicate things even more, a third of the plants are under old trees, and one zone is half shade and half full sun. Not a good design, but it’s too expensive for me to change the design at this point. So my goal has been just to make sure the annuals and the hydrangeas don’t wilt and just hope the rest of the plants will do ok.

Any recommendations for setting my zones up would be greatly appreciated!

@stonecliff, I’d configure the zone to water for the shrubs with shade. This way you’re watering to meet the needs of both the perennials and shrubs root zone depth, then the water that’s left will get used by the trees. If you believe more water is needed, you can increase the crop coefficient for the zone in the advanced settings.

@Linn, annuals have a relatively shallow root zone depth. I’d recommend the same settings with partial sun to account for the mix solar exposure mentioned.

Many thanks @emil. I was wondering if that is the variable that might be the one to do.

Can you provide a brief lay interpretation of what the “crop coefficient” actually represents?

The crop coefficient (Kc) is a ratio adjustment factor for evapotranspiration (ET) for when plants are expected to produce optimum growth and/or maximum yield. The crop coefficient is determined for each kind of crop under very controlled conditions.

I found a great article on it at Plant Factor or Crop Coefficient What’s the difference on the University of California site.

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Thanks, @emil. I’m glad to have the confirmation that setting the zone for the plants that have the shallowest root depth is the best way to go. Last year I only had the Rachio on for half the watering season, so I’m really excited to have it on all season this year. Based on what I saw it do last year, I’m pretty sure it’s going to perform way better than my old Rainbird system (I am using flex with these drip zones).

Thanks, @Linn. Good article. Seems like the more I learn, the less I really know!

Now it’s got me thinking about calculating my own crop coefficients by month and type of plant!

@stonecliff, now you are starting to become an expert! At one point in my life I used to be a mainframe computer systems programmer, and then a manager of them. I used to tell my staff that you really weren’t a good systems programmer until you knew so much that you realized you didn’t know anything!!!

I’m sure @plainsane would be a good person to talk to as it looks like he has played around with the crop coefficients a bit. I do know that from the reading I did it is a very controlled scientific process, so I would play with it very slowly.

Another lesson learned from my years in computers was to only make one change at a time, and watch what that one change does. If you make two or more changes at once, you are never sure of what caused what to happen. I think that is especially true for those of us trying to tweek the settings.

It’s fun to do,what rachio’s canned vegitation is set to is good enough.

Yea, that is why you can earn Ph.D. In these areas now. Look at pwp, it is surprising how squishy that concept is.

Micah woods is a good source to follow, pace turf, dr. John Kaminski at psu. They are heavily turf focused though.

Personally I computed my et and it was not worth the effort. It’s a heavily involved process that you can google. I had computed 5% higher than rachio and could not tell a difference with their default (could not adjust it way back).

But in short, the et is the rate at which a plant moves water from the ground into the roots then through the leaves. Most of the water is separated via photosythesis to get the hydrogen for constructing chloroplast and cellulose while releasing the oxygen. Some water is just released for cooling purposes.

Infinite monkey cage has a fantastic podcast on this process, to show just how little we really know about the whole process and to highlight why there is growing need to understand quantum mechanics. It appears that plants utilize quantum adherence for photosythesis, which compared to solar cells, are crazy efficient. It’s a very bizarre podcast but I like Brian cox a lot as he is the closest thing we will ever have to a Carl Sagan imo.

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You will always struggle with this. The root canopy of a very mature tree is insane. It will always soak up the water.

If you have drip irrigation on the shrubs, I would first try lowering (shallow) your root zone (not lower than 8),to force the scheduler to increase the frequency of watering while shortening the duration ( we have a competing root mass here). You will have to make the call on how much shade the shrubs get but keep this honest as it will define the ratio for the solar effect. I personally would move your crop co up a bit to a 75% (to start) since you have a competing root mass and if tje plants need more water even still , do it by first moving up your co as this will have a stronger relative impact as ambient temperature rises through the season. If this not enough then the root zone is not deep enough so increase that to get longer watering. (On co, don’t go over 95% I don’t know their algorithm but 100 means evap * 2 and and means another part of the model is wrong)

If the soil has moisture say 3 inches from the tip of the shrub, a day after watering, then you can move that co down, close to 50.

I have done this for my vegitation under my bald cypress (my co is 65%, root depth at 10 with 2gph emitters run zone for 40) and they don’t have the best root ball but the plants are thriving. The bald cypress builds amazing root structures.

If you have spray head, well, enjoy your new swimming pool or take out the shrubs cause you are gonna throw a lot of water.

Here is one study I read on root competition that might help as a jump off for more specific crop studies.


I specifically left out altering mad because we want to get to root dialed up. The ultimate last ditch method to increase frequenc is to move mad down ward, but first trying to dial in the root zone first is the best route as it provides more real data to the model.

The most accurate config here will be a mix of all three adjustments