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Plant Focus

A guest post by Matt Candeias, host of the In Defense of Plants podcast and blog

Flood-resistant Oaks?

Last spring (Southern Hemisphere) Grigadale Arboretum was partially flooded. A similar situation had occurred in 2002 and the historical record shows a comparable flood in 1923. The 2002 flood was disastrous for the park and surrounding farm, with many young trees lost, including some oaks planted by my father. At the time it was said that this sort of flood was a "once-in-a-generation" event. We must now accept it is at least a "twice-in-a-generation" event, if not more, especially if the increased flood frequency is due to human interference, either in natural draining systems, through climate change, or other meddling with Nature. 

In 2002 my parents made a list of the trees that made it through the flood, which involved trees being under water for several months, from summer through spring, and those that didn't. As far as oaks were concerned, these are winners and losers:

Quercus ilex
Q. ellipsoidalis
Q. rubra 'Aurea'
Q. aegilops var. pyrami (now Q. ithaburensis subsp. macrolepis)
​Q. velutina
Q. ×ludoviciana
​Q. bicolor

Q. acutissima
Q. palustris
Q. suber
​Q. nuttallii (now Q. texana)

Q. texana in the foreground. Photo taken last November from a boat on the lake in front of the house. The lake has expanded considerably due to flooding. See photo below for what this view normally looks like.

Q. texana in May 2014. Photo taken on terra firma. The lake begins out of frame to the right. 

Q. acutissima seems to adapt particularly well to our conditions in general, its acorns producing legions of seedlings under the mother tree, but I wonder if there is evidence elsewhere of it being resistant to flooding. Q. palustris is known locally as roble de los pantanos (lit. oak of the swamps, in tune with its Latin epithet and similar to one of its common names in English, swamp Spanish oak), suggesting that it would not be bothered by being under water. Q. suber is a surprise survivor, but it is in a location that would not have been under water for the duration of the flood, only the worst of it. Q. texana survived well, and notes indicate that it spent 8 whole months in water. It spent a few months in deep water last year as well (see photo above) and seems none the worse for it.

The area where oaks were lost in 2002 was near the lake in front of the main house on the property, a natural depression that would normally form a small lake in wet years, and that is artificially maintained as a landscaping feature by means of excess run off from a windmill and an electric pump when required. This area is lower than others and so prone to flooding should flooding occur. However, confident that a flood was a freak event, I planted several new oaks in the same places where my father had lost his, and learnt the hard way which other species do not like having their feet in water for extended periods.

My list of lossesand those that made itall 2- or 3-year-old trees:

Q. ellipsoidalis ‘Hemelrijk’
Q. coccinea
Q. velutina ‘Oakridge Walker’
Q. shumardii
Q. ‘Mauri’
Q. ×jackiana
Q. petraea ‘Purpurea’

Q. lyrata
Q. phellos

I had recently installed drip irrigation for the oaks in this now-to-be-considered flood-prone area, so am loath to quit just yet in the attempt to establish oaks there. However, it may be a good idea to only plant oaks that may survive flooding, rather than rely on the false promises of statistics and probabilities.

So, which oaks might be flood-resistant? My first thought was to search in oaknames.org for oaks with “swamp” in their common name. This is the result:

Swamp chestnut oak (Q. michauxii)
Swamp laurel oak (Q. laurifolia)
Swamp post oak (Q. lyrata)
Swamp red oak (Q. falcata, Q. pagoda and Q. shumardii)
Swamp Spanish oak (Q. pagoda and Q. palustris)
Swamp white oak (Q. bicolor)
Swamp willow oak (Q. phellos)

Although two on this list have already failed our flood test (Q. bicolor and Q. shumardii), three of the survivors are among the “swamp oaks” (Q. lyrata, Q. palustris, and Q. phellos). Our star survivor (Q. texana) is not known as a swamp oak, but perhaps those who know its natural habitat better may be able to comment. One of its common names listed is “red river oak”. “Water oak” would presumably be another common name to pay attention to. According to oaknames.org, water oak is the common name for Q. nigra, “as well as colloquially for Q. arkansana, Q. falcata, Q. laurifolia, Q. lobata, and Q. palustris.” So that adds some more options.

A closer look at Q. texana standing in about a meter and a half/5 feet of water.

And perhaps even better would be to choose hybrids of these hydrophile oaks, in the hope that hybrid vigor would compound inbred waterproofing (though the demise of Q. ×ludoviciana (Q. pagoda × phellos) appears to suggest the contrary). I have seedlings from Nativ Nurseries (see Mississippi OODs in Oak News & Notes Vol. 18, No. 2) of Q. pagoda × nigra, Q. shumardii × nigra, Q. shumardii × phellos, Q. lyrata × Q. michauxii (Q. ×tottenii), which all are “swamp oak” hybrids, and Q. lyrata × alba (Q. ×beadlei), Q. lyrata × virginiana (Q. ×comptoniae) and Q. lyrata × alba, which have at least one “swamp oak” in the mix. On the other hand, we have already had two of the half-hydro-hybrids sent to Davey Jones' Locker (Q. ×jackiana has Q. bicolor as a parent and Q. 'Mauri' has Q. palustris paternity), so here again, one must hedge one's hopes.  Nativ Nurseries call their Q. lyrata × alba hybrid ‘Rainmaker’, so perhaps that is one to avoid… (Though I am not sure why they chose that name – Dudley Phelps, if you read this, please provide the answer in a comment below!)

Plenty to choose from, in any case, if this is a good strategy for planting oaks in an area susceptible to flooding. I am sure other IOS members will have knowledge and experience with these species and issues, so I look forward to informative comments.