Cause of root/crown rot of perennials in greenhouse?

7 years ago

At college we are participating in the Philadelphia Flower Show, and we've been starting a bunch of perennials in the green house which includes a a few varieties of Heuchera, Ajuga, Coreopsis, Hostas, etc. So far, we have lost about 60% of our perennials, all due to root or crown rot. Most of them were started in January, after keeping them outside for dormancy. The green house is kept at about 80 degrees, and the plants have grow lights on set timers, and there is no shade cloth on the green house. One problem is that the soil seems to be staying moist for a long amount of time, although the perennials came from three different nursery's and some have been re-potted.

Our instructor is a certified Horticulturalist who's been doing this for years, and he's blaming the soil medium and the weather (warm and cloudy winter). There was a lady who visited our greenhouse and said we are overwatering and should be using shade cloth. I think it might be ventilation. Our instructor though is extremely stubborn and one of those guys who can't ever admit he's wrong, even if there's an obvious solution. So I'm trying to see if anyone here might know the solution...

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Comments (5)

gardengal48 (PNW Z8/9)
7 years ago
Root and crown rots are almost always caused by excessive moisture. So it could be a combination of both the potting medium being excessively moisture retentive and watering too frequently. FWIW, a very textural, fast draining potting medium significantly reduces (but not necessarily eliminates) the issues caused by over watering - I'd look to that solution first.

btw, your instructor seems to be the one that arrived at the most logical cause. I would not be inclined to doubt him :-)
User
7 years ago
last modified: 7 years ago
Over-watering. Always, for me, this is the main cause of attrition and loss at this time of year.. I topdress the pots with fine grit so it can be difficult to tell if the plants are too wet or dry without lifting the pots...and being an idler, I tend to skip this for every single pot and just chuck over extra water (nearly lost several pots of gypsophila 'Kermesina' today)...often only noticing when the plants go limp and stringy. Tipping the pots out should be a good indicator that something is amiss.
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I guess I missed an obvious comment on this... I don't think you should assume you have Crown Rot... you might just have a rotted crown. As Ken points out, hostas can rot if they are not handled well... cold, wet soil... buried too deep... heavy soil in a pot keeping it too wet and hot... lots of things can get a hosta to rot in a pot. And then there is Southern Blight... which from the descriptions I've read over the years may be similar but there may also be a particular kind of fungus involved... otherwise there wouldn't be the advice about discarding soil and such... in any case, Zilis suggestion for dealing with it, from the Handbook is "dig infected plants... remove any leaf tissue, and soak clump in a 10-20% bleach solution for 1-2 hours, repot and isolate for 3 months, remove any soil that would have been in contact with the affected plant; in nursery settings spray or drench with fungicides once symptoms appear." and one note... comments like Zilis have lead to considerable confusion about bleach solutions. The bleach we buy in the store is already diluted, as you will find by reading the label. Ignore that. When a gardener says a 10% bleach solution, they are saying take your store-bought bleach and mix it 1 part to 9 parts water...
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overwatering, too "heavy" a soil mix, planting too deep... and and all could be the culprits. Since the plant that is still in the ground probably has an open wound, or young calous, you might improve its chances of surviving winter by dusting with a little fungicide. But take a look at it - if the crown isn't up at the surface with the soil slanting away from it a little - I'd raise it. Often when this has happened to one of mine in the garden - or more often in a pot - I've found that the hosta has sunk a little so when watered, it ends up sitting in a little private pond area, which encourages rot and dismemberment of the crown. BTW - I've grown some EW... I think it is 30 now... in pots and in the garden and I've found it to actually be a pretty sturdy hosta... which just makes me even more suspicious that it is too deep and in soil that is too heavy...
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Being careful with bleach is more for the human side - where the risk of eye injury is greatest. General use of bleach demands eye injury awareness. We have noted in this forum for years the use of dilute bleach has not caused any ruin in our hosta growth for +50 years but IT will kill almost all fungal diseases if physically contacting the spores or organisms. Having worked in the microbial biologicals business for years; the quite standard and common way to sterilize water and equipment ( and plants!!!) in hospitals, fermentation, food processing plants and plant nurseries is bleach. Not anecdotal but standard operating procedure!!!! http://ucfoodsafety.ucdavis.edu/files/26437.pdf Yes, if you leave a plant or root mass in warm/hot water with bleach present , it will soften the root tissue but if you rinse in cold water and cool back down - no foul! Yes, if you let it sit for 12 hours in hot bleach solution or in the sun you will kill the fungus and potentially the hosta plant. But it is the only effective practice to kill fungal spores and fungal bodies/rot. Not sure what is meant by "roots starting to rot away from the bleach"; you will kill the tissue if temp of water too high or time exposure too long definitely. I can safely knock down southern blight using dilute bleach on the plant, remove dead plant tissue and the plant recovers. Bruce
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I should mention that it looks like it does because I peeled the dead outer bark away to see the extent of the damage. The outer bark started to crack, flake and slough off. The inner bark was a combination of crumbly and mushy. Dry underneath that. The brownish pieces scattered in the lower right of the photo are what I found underneath. No signs of borer tunnels, insects, or frass that I am used to seeing.
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ken_adrian Adrian MI cold Z5
7 years ago
there might be a greenhouse forum also ...

80 degrees sound ridiculously high .. imo opinion .... if you are trying to replicate spring growth.. whose spring jumps immediately to 80 degrees??? ... not in my MI for sure ... no clue where you are ... [probably near philly.. lol..eh]... btw... you seem to be spot on with the ventilation suggestion ... as that is how you would reduce temps ....

next i wonder why you need 24 hour light ... nor grow lights during the day ....

also .. were they divisions which were divided... another source of rot ... especially in wet hot circs ...

no clue if you left them outside long enough and they actually achieved the requisite chill dormancy, if any .... which also brings to mind.. if you brought them from cold.. to 80 degrees with no hardening off.. another rot potential ...

as to watering... the best grower i ever met ... never watered.. until the house started to wilt ... i was stunned to see various houses looking like death ..

he said its the best way to insure media drying ... encourage roots to stretch out to search for water ... etc ...

also.. dont forget excessive fert is another source of rot .. when miss timed.. applied to cut tissue ... etc ...

frankly ... im a novice.. and i think your dude may have done everything wrong ... but dont tell him or her that ... lol ... i presume you want a good grade ...

ken
gardengal48 (PNW Z8/9)
7 years ago
Ken - forcing plants to bloom out of season, like what most commercial flower shows offer in their bizarre and unnatural display gardens, DOES require simulating the same conditions the plants would receive growing normally during their usual bloom period, like higher temperatures and strong light, and this needs to be condensed into a few short weeks, not a full growing season.

Let me be perfectly clear - crown and root rots are water-related issues, either from over watering or excessively moisture retentive soil or a combination of both.
pennlake
7 years ago
80 does seem hot, even for winter forcing of temperate perennials. Producing a quality plant is a balance between temperature and amount of time it will take. Hot and wet is likely to produce soft, stretchy plants. If your media is not drying out very something is wrong. Another easy way to kill some perennials in the greenhouse is to plant them too deep combined with overly wet media.

Cause of root/crown rot of perennials in greenhouse?

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gardengal48 (PNW Z8/9)

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