Frost Dates and Irises

Keep seeing everywhere folks asking about how late to plant irises. Some experts say 30 days, or 6 weeks, or 60 days before frost.

It has occurred to me that I had to move to the PNW before I'd ever heard of "frost dates". Anyone else here not know to what we are referring? Any newbees wondering what that is?

it's the "average first frost date" in fall and "average last frost date" in spring.

For planting irises, we're talking about the first frost date. Your local gardening group, or your county extension agent, will know what the average first frost date will be. For example, our average first frost date in Wake County NC is Oct 15th, even though we don't get killing frosts most years until sometime in November.

For TB irises, I've always heard irises need to be in the ground a minimum of 4 weeks before your area's average first frost date so they can develop the roots they need before winter. some growers will not guarantee plant survival without that length of time and they close up shop in August to help prevent problems from folks wanting to plant too late. I'm not sure, an expert needs to help me here as I'm a bit of a newbee myself, but I think for the beardless irises, it needs to be more? or less? than 4 weeks.

As I learned the hard way, that's an average date, so if your area has an earlier frost predicted, you may be scrambling to cover plants! been there, done that, too.

This message was edited Sep 8, 2008 3:11 PM

IN 1987, we moved from here to Idaho Falls Idaho. That's when I learned that I would have to be very careful what garden seeds I chose, because their average last frost date was June 15th and their average first frost date was August 15th. And just to prove it, it snowed on my sunburn the last week of April when we arrived.

sometimes rebloomers can be hit by frost. When we had them we could put a bucket over the small ones at night & they would be fine for another week. a friend used a stepladder covered by a sheet of plastic.

I took the thin metal bars we use to hold up Christmas lights along our walk and used those to keep blankets and thin tarps up off the plants a bit if the rebloomers had stalks.

Last year I had a stalk of Double Shot on my Thanksgiving table.

Last year I purchased 4 iris from VB and received them October 5th. October 15 was our first frost. 3 of the 4 bloomed this year. Don't know what to say.

it varies a great deal from iris to iris. last year I planted until Nov 1st, and we had frost the next week, but I covered plants. And most bloomed. But the "30 days" guideline is the best for your plants. SDBs are a lot hardier than that.

I checked with Cornell University and my average first frost date is October 20 - 30 give or take a few days. So if I plant in the next week maybe 10 days I should be okay. We have had pretty warm Octobers here lately. Last couple of Halloween's we were able to go outside in short sleeved shirts. When my children were small I remeber SNOW on more than a couple of Halloween's!! Times are a changing I guess.

Another thought on this. Mulching the iris's over the winter with leaf mulch should help them survive as well right? Extra protection?

irisMA - you'd best answer this last about mulch - all I know is we don't mulch. irises like the cold and snow, and we barely get enough.

Frost and Iris - Remember this is my humble opinion! Based on what has happened here in Maryland.
Yes you should plant your iris before the frost.
Reality - I have planted into November and had snow on the ground most of the rest of the winter - Yes I had heaving but having a bucket of Miracle-Gro potting or garden soil in the house can help with the, just put it on and gently tamp down.
The guide line to get iris into the ground before frost is GREAT advice and can help prevent heaving ( rhizome comes out of the ground exposing roots), but there are always something that comes up or those last minute moves you have always wanted to make. If the soil is thawed and workable you can plant, move, dig iris.
Will you loose some? You will anyway, you are taking a stressed plant, dug, bleached, dried, shipped, put in new soil with different composition and probably treat it different that it had ever been treated before. That is stress, and yet even with this 95% of iris come back just fine. You might loose a small percentage more planting late but I have not seen a hugh difference based on planting time.

:) Anita

As I mentioned before, I plant right into December. But with the snow cover Debbie and I get they are well insulated, and I think we can still plant for some time.

That's not true in all areas. And also I understand the frost heaving can do quite a bit of damage, and we don't normally get that in upstate NY.

This message was edited Sep 9, 2008 10:10 AM

Using leaves for winter mulch can lead to rot as they hold water. If that is the only material available run the lawn mower over them several times, but here is the catch, get the "mulch" in between the rhizomes. The main reason for a winter mulch is to prevent heaving, common with the bearded iris. If you are cold enough so that the ground stays frozen all winter it shouldn't be necessary after the first winter. Our ground freezes & thaws throughout the season & we grow many smaller iris plants which are more apt to heave. Last winter we put fist size rocks on the top of the rhizomes which arrived late from another climate. Bricks will do the same job if you don't have New England's supply of rocks.

Thanks for the tip. Maybe I'll just leave well enough alone this year.

Maybe you will have good snow cover which should solve the problem.

Polly, I can still remember being a 10 year old kid in Poughkeepsie, and building a snow fort 12 feet tall out of snow blocks we carved with mittens out of snow that was two feet over my head .... and that was in LOWER NY - LOL oh yeah, y'all get all kinds of nice insulating snow. that snow fort was cozy inside, too!

But like, in IF ID, it's so dry all the snow was powder and would blow away and the ground would freeze solid down to 8' deep, -43 my first 1/1/ there.....my eyeballs froze.

So everyone has their own microclimates.

anita, IrisMA, polly, any of you want to chime in with good "estimates" or guides for cut off dates for planting the beardless iris?

darn it, anita, you just reminded me I just found my certificate for Karen Jones from the May auction in the attic yesterday and what with my car dying I forgot about it again. Wonder if it's sold out yet? Ginny and Don still teach school, don't they? maybe I'd best call tomorrow afternoon after school. Be at Plant Delights Nursery in the morning anyway.

Bonjon, thanks for asking. But I'm not really a bearded person, and only know what works for me. I posted here as Debbie is close to my area, and should have about the same growing conditions. Our snow is wet and heavy and sticks, BTW.

Two years in a row I have been planting irises up until December.

Also I have a nursery, and have been purchasing tall bearded for sale for around 14 years. I buy them wholesale from Walters Gardens, and then pot them. In about the end of October I put the gallon pots in a trench, and overwinter them that way for sales the following spring.

I retired two years ago, and three years ago I started buying bearded for my own enjoyment, planning on having the time to care for them.

So I have 14 years of knowledge of potted iris growing, but only three years of growing in the ground. So I'm not one to be able to give much advice on bearded.

I know a lot about beardless, though, so any questions on them, ask away, LOL.

We have done the "pots in the ground" bit with bearded--also the rock on top of the rhizome bit, but not after Sept. As I remember Raleigh is at altitude so I wouldn't go directly in the ground after mid Oct. Listen to Polly about beardless.

LOL, IrisMA.

A near miss 36 degrees right now I'm sure people north of me got a frost.

have this mental image of a bearded polly...no, my question was about beardLESS, and you taught me about pots and winter today!

Raleigh has altitude? what altitude? Idaho Falls was at 4700' - now that's altitude. we're in starting edge of piedmont, I guess. south end of county is flat sandy soil like the coastal areas, north end of town is all hills and clay soil. we might be 800-1000'. being a capital town, I'd say we have more attitude - but that's probably a better description of Chapel Hill (giggle)

are you closer to sealevel, irisMA?

bonjon I believe that you could probably put beardless in the ground pretty late in NC, with these remember that you can kill them if they are not established. they truely need the 6 weeks. I would think mid october - to 1 Nov for NC except the mountains.
I will d-mail you with an e-mail address of some iris people around charoltte (??sp)
Anita

I am planting siberians, japanese, spuria and daylilies to mid Oct...in zone 7. We have gotten down to high 30's at night.

When we first planted the siberians it was in mid November, raining, and we had freezing weather at night...lost 4 or 5 clumps and that is all. Some did not multiply or grow as well, but we had no other chose it was then or never.

I did get a ton of cheep composted STEER manure and crowned each clump with about 3" of it and that helped.

D

Sorry, Bonjon. I need to read slower.

And no beard yet, but I am getting older.

I would say a cut off for your area for the hardier beardless like sibs would be early November. I would want JIs and LAs in by mid October. Mulch them all really well, and keep moist until your ground freezes, not just first frost. Don't fertilize when planting.

Altitude about 50' although we have a 100' plus ridge between us & the coast. I was thinking of Raleigh as opposed to the NC coast not height in the grand scheme of things. Attitude? You should see Harvard.

I HAVE (Wellesley girl)

ROTFL


I liked the boys at MIT better

and Polly, I'm old enough to already resemble my remark! (giggle)

This message was edited Sep 10, 2008 6:34 PM

Just planted beardless 'Holden's child' rescued from shade today. Replaced pseudacorus with it. something is scratching up some SDBs, while have to use the'rescue rocks'.

United States Average Zone Frost Dates
Zone 1

Average dates the last frost - 1 Jun / 30 Jun

Average date of the first frost - 1 Jul to 31 Jul

vulnerable to frost 365 days per year

Zone 2

Average dates the last frost - 1 May to 31 May

Average dates first frost - 1 August to 31 August

Zone 3

Average dates the last frost - 1 May to 31 May

Average dates first frost - 1 September to 30 September

Zone 4

Average dates the last frost - 1 May to 30 May

Average dates the first frost - 1 September to 30 September

Zone 5

Average dates the last frost - 30 March to 30 April

Average dates the first frost - 30 September to 30 October

Zone 6

Average dates the last frost - 30 March to 30 April

Average dates the first frost - 30 September to 30 October

Zone 7

Average dates the last frost - 30 March to 30 April

Average dates the first frost - 30 September to 30 October

Zone 8

Average dates the last frost - 28 February to 30 March

Average dates the first frost - 30 October to 30 November

Zone 9

Average dates the last frost - 30 January to 28 February

Average dates the first frost - 30 November to 30 December

Zone 10

Average dates the last frost - 30 January or before

Average dates the first frost - 30 November to 30 December

Zone 11
Free of Frost throughout the year.


This message was edited Sep 14, 2008 10:32 AM

I shouldn't say "overkill technically" after over 40 years as a technician but, in
a lot of instances, too much emphasis is placed on being technically correct.
As I had said before on another thread, more irises are killed by kindness than
by neglect. I am a worst offender in that respect. Too much water, too much
fertilizer and too many trips around the beds trying to encourage faster growth.

With that said, my best results have always been PLANT EARLY. I have some
irises which were sent to me in late July that are 12 inches tall now. Some of
the dwarfs are not only putting out the normal center of fan leaves but are also
sprouting their increases. September is the critical growth month in my area
because it is past the hot and dry July/August period and we begin to get the
cooler rains.

I have also kept TB rhizomes in a refrigerator crisper all winter and planted them
early in the spring. 99% survival. In colder climates, plant them asap and cover
them with a mound of soil until spring when the freezing is over then remove all
the extra soil.

They will form roots and new growth at their own pace and nothing you can do
will speed up the process. Just keep them from heaving out of the ground. They
are probably among the toughest plants around the garden.

Probably a lot of wind from an amateur but I have always had a tendency to
analyze any process before taking action. You can over analyze and make
things a lot more complicated than necessary. Other opinions welcome.

Question:

About 3-4 weeks ago I planted some rhiozomes from the iris swap....
Since we've had all this rain and its really mushy back there now are they
going to make it? It's sapposed to start drying up Tuesday with lots of sunshine
throughout the rest of the week....I just hope they won't turn to mush too ughhhh.

Delane

those are some awesome clumps Delane.

keep an eye on them if they are in an area that's not draining well.

Thanks bonjon ;o)

It looks like it drained all off...however I hear were sapposed to get rained on again...
It's not rained like this in the 6-7 years we've lived here...residuals from Ike and all the way up here?...These rains would have been awesome in spring and summer as we needed them...we have severe flood warnings here too.

I seperated those clumps and I have 3 more to go....they are beautiful and smelll
sooo good...they are from my great aunts garden some 40 years ago. I wish I knew the name of them...They are so hearty and healthy and an awesome grower they take right off.

I wonder if they have started to root yet? I just don't want to lose them.

Rechecked my temp beds & found 3 that had 17+ inch leaves. New growth
from late July planting.

It appears that they are on a good slope so drainage should not be a problem.
Would you, by any chance, have a closeup pic of one of them? Vigorous white
fragrant iris 40 years or more old sounds like an antique possibly diploid iris
I am really interested in. I would be interested in some trades next year if
you have a surplus. I am trying to build a collection of the oldies.

Planted in July blooming today! "Breakers"

My info says 'breakers' is a rebloomer. Were the rhizomes freshly dug or older?
When I replant same day digs it seems they take off growing immediately but
older dried ones seem to take about a month to really activate.

I have a 'Lo Ho Silver'?? reblooming now but it is about 3 years old.

Yes its a rebloomer, fresh dug planted a few days after that. Total recall is still reblooming for me as well. Neither bloomed this past spring though.

The freshness of the rhizomes makes a big difference in the bloom cycle for
me. I am sure all the folks in a colder climate would be more severely affected.
As far as frost dates are concerned, the rhizomes are on top of the ground and
are certain to freeze regardless of whether they are planted late or early. My
problem was with heaving. I would find new rz's sitting on roots as much as an
inch above ground. That is why I recommended just dumping 3 or 4 inches of
soil on top of them early in the winter then take it all off in the warm spring. One
lady said put a rock on top of them. Yankee ingenuity.

Back to reblooming: I had my LHS?? try to bloom the first week of November
last year but the buds were killed by a quick freeze. Anxious to check again
this fall.

I don't have problem with heaving in my lower gardens because I use sawdust for mulch keeping it just away from the rhizome and my upper beds generally have much more snow on them from me shoveling my driveway so I usually don't have a problem with those either being its probably the last snow to go in the spring.