Pollarded Birch bleeding sap

Last year I coppiced several of my birch and got a moderate survival rate. This year, to try and improve the survival, I decided to pollard them instead, and cut at about 3 foot high. I cut them down about three weeks ago, and I thought they were pretty dormant then, but yesterday I noticed that several were oozing moisture from the cut surface.

Birch with sap bleeding from cut trunk

How worried should I be: is it going to kill the tree?
Will the sap dry up in time, or is there something I should do to try and stop it?
Does this mean I could harvest sap from my other birch trees at the moment?

Nancy Reading wrote:How worried should I be: is it going to kill the tree?
Will the sap dry up in time, or is there something I should do to try and stop it?
Does this mean I could harvest sap from my other birch trees at the moment?

I think it's one of those 'garden according to what Nature is doing' situations?  

But also, apparently, in a mild winter, birch sap can start rising in the UK from early February apparently, or in a cold one in N. Scotland, late April, apparently.  So apparently birch are best pruned in late summer or early Autumn: https://www.rhs.org.uk/plants/birch/winter-interest

Stump diameter and age affect coppicing of downy birch (Betula pubescens Ehrh.)

"The effect of stump diameter, stump height and stand age on the sprouting of downy birch was studied by clear-cutting six stands from three age classes (A: 10–12 years, B: 15–16 years, C: 22–24 years) located in northern Finland and measuring the sprouts in the following autumn.

"The percentage of non-sprouting stumps increased with the stand age (A: 9%, B: 14%, C: 27%).

"The smallest (< 1 cm) and biggest (> 9 cm) stumps showed higher mortality than medium-sized stumps."
https://link.springer.com/article/10.1007/s10342-019-01175-5

Several sources recommend against pollarding birch, although the above seems to suggest it can work. It might be worth tapping the sap, in case the pollard doesn't make it?

Thanks AC. From your first link:

When pruning is required, prune only when fully dormant from late summer to before mid-winter. Birches bleed heavily at all other times, particularly in late winter.

Hmm, I must have been lucky last time then, since I thought I coppiced at about the same time as this year. I was thinking anyway that I ought to aim to get my cutting done before the end of the year, to allow for tidying and stacking before the weather improves, and I want to be doing other things.

It might be worth tapping the sap, in case the pollard doesn't make it?

I might see if I can rig something up. How to seal a bag over the end.....?
I did swipe a finger across the stump and the sap doesn't taste sweet at all. I think birch sap is something like 100x less sweet than sugar maple, but I remember having rather good birch sap wine once, although maybe they fortified the sap. I seem to remember reading that it is very toxic for dogs (contains Xylitol), so will have to watch Dyson and Della don't get their noses in it.

Nancy Reading wrote:I think birch sap is something like 100x less sweet than sugar maple,

The numbers I normally see kicked around are something like: sap from sugar maple needs to be reduced 40x and sap from white birch needs to be reduce 100x. Might that be what you're thinking of?

Thanks Christopher. Well if it only takes about twice as much reducing as maple that's not so bad! We have the wood stove on most of the time, so I can leave a pot on top evaporating away..... I wonder if there is a way of encouraging evaporation as the sap is being collected - you can see a whitish deposit as the sap has run down the trunk in my photo in the first post. Presumably the sap is evaporating, leaving a residue behind. I don't think I can rely on the weather being as kind over the next week though.

You can put wax on it.  But maybe the sap is sealing itself?  I'm in a different climate, but where I am, it takes a few days for the tree to produce a protective layer over the wound.

That is unless it's frozen within 3 days of the cut, then it takes longer and I might try to find where I left my pruning wax.  

Nancy, did the trees make it all right or did the ooze lead to something else?

Thanks Timothy!
The ooze stopped after a couple of months. There did not seem to be any infection or other detrimental effect to the trees. All of the pollarded birch survived to regrow. About 1 in 6 of the coppiced ones failed the previous year, so since I only cut three birch last year the sample isn't really big enough to tell whether that is any different. I never noticed any bleeding on the coppiced birch stumps.

I haven't been particularly happy with the pollard's growth though - probably unrelated. The new growth appeared to be rather brittle, as I had several of the new branches snap off either at the trunk or a few inches up. I think the first year after cutting a larger tree (whether pollarding or coppicing) appears to be the tree replumbing it's growth patterns from the previous branches to new sprouts. The second year is when I see how well it is going to do, so I'll take how much they grow this year into account too. I coppiced just a couple of largish birch this year, but did not pollard any. I cut them more than a month earlier - sometime in January - to avoid bleeding issues.