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Drip irrigation and drought tolerant plants

Posted by alison_sunnyday 8b coastal WA (My Page) on
Tue, Jun 21, 11 at 11:50

I moved into my home here on Whidbey Island two years ago this summer. The house has over 100 years of history and the gardens reflect many owners' inputs and additions. There's an extensive drip irrigation system in place for watering, but it shows little rhyme or reason.

Today's project is to tweak the drip system so it quits dripping on bare soil and starts dripping on plants that need irrigation.

My question is, should my Ceanothus, lavender, spring bulbs be excluded from the drip system? We get some *really* droughty summer stretches here in the rainshadow and I'm wondering if I shouldn't leave a single dripper on at least some of the plants (I run the system for 1.5-2 hours and they're 1/2 gph drippers). Oh, how about Germander/Teucrium chamaedrys? I have a ton of it, and some is already excluded from the drip system and still doing fine.

How about established trees and tree-like shrubs? I have some Rhododendrons, Buddleja, and a Dogwood that aren't blooming well, as well as a Katsura tree that suffered a lot of die-back but is recovering, and I suspect insufficient moisture as one contributing cause -- I was figuring on upping them to 2-3 drippers per plant?

Thank you so much for any input you can offer! Finally this is the year I put my *own* stamp on the gardens here. :)

Follow-Up Postings:

RE: Drip irrigation and drought tolerant plants

My first question is, did you run the drip irrigation system before and if not how did the plants do last summer? Anything that did fine through last summer ought to do fine again.

Consider where the various plants you mentioned - and others in your garden - are native. Ceanothus, lavender, germander, and many spring bulbs are native to parts of the world with dry summer climates. They should probably not be watered in summer certainly not if they haven't been in the past. Rhododendrons, dogwoods (unless our native ones) and katsura are all native to east Asia which has very wet summers. You probably can't go wrong watering these deeply and regularly throughout the summer. Most Buddleja hybrids in cultivation are mostly of east Asian origin as well but are still somewhat drought tolerant because they are adapted to tougher habitats in the wild.

1/2 gph, that doesn't sound like much. I'd run them a lot longer and/or put multiple drippers per plant, or come up with a new system entirely. It is best to water infrequently and deeply - like, once a week - than dripping a little bit on them every day. The exact amount of water you want to put on plants depends on how large/established they are, but 1/2 gph really sounds low to me to do any good.

RE: Drip irrigation and drought tolerant plants

Thanks so much, Ian, I appreciate the logic!

I ran the system last summer, but there were a lot of breaks in the line that I think made the pressure throughout the system extremely variable, plus I ran it for way WAY too short a duration each time, so previous performance is really not a great gauge. The surviving ceanothus are blooming well and have new growth but all have some number of mysteriously withered branches.

I've currently got it set to run 1.5 hours twice weekly. I might up that to 2 hours, plus I plan to run multiple drippers to the larger plants as part of the current overhaul, and may give some small but thirsty plants 1 gph drippers instead.

Still not sure what I'll do about the lavender. My gut feeling is that supplemental water might not hurt but probably won't help a ton, unless there's a *really* droughty spell. So do I save on the water bill by excluding them from the irrigation? Hrm.

RE: Drip irrigation and drought tolerant plants

Withered branches on Ceanothus could very well be freeze damage from last winter. Around here many Ceanothus are damaged and even killed that had gone unharmed for many years previously. Prune the dead parts out and they ought to look fine again soon.

Lavender, once properly established (unless it has developed a shallow root system from being watered too much throughout its life), ought to be able to handle our summers without any help. My approach is if you can't take the drought, get out of the garden - something like that. Especially if you're paying for water!

RE: Drip irrigation and drought tolerant plants

Ceanothus should not be irrigated once established. Neither should spring bulbs. You think you have droughty summers, what would you call California's 9 month long dry season? that's where most ceanothus come from, and the ones that are native up here grow in the drier areas like rocky balds on the dry side of the Olympics.

Withered branches on ceanothus could well be freeze damage, or could also be root rot from being irrigated in summer.

Watering less, but just as often, isn't helping drought tolerant plants. It would be better to water less often but deeper. So leaving a single dripper on a drought tolerant plant, while continuing regular watering over the summer, won't help and could well kill.

Lavender is a toss-up as to whether it needs watering or not. They're short lived anyway so don't worry about them. Germanders do not need watering.

Rhododendrons do need irrigation over the summer, but a deep watering once a month would be enough for a large, well-established rhodie. They don't bloom well if they're not watered over summer. Katsuras and dogwoods will want water too but why bother irrigating a buddleja? they're weeds anyway, and native to dry gravelly sites. Spreading throughout western Washington now too, where they're definitely not getting watered.

Any plant on a drip system should have more than one emitter. The water doesn't spread all that much so you should have at least three in my opinion, spaced around the plant. I use drip tubing and make a ring around each plant. And keep in mind that the roots are not next to the base of the plant but quite a ways out, and you need to put the emitters where the roots are.

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