Hello Kathy,

Unfortunately (but predictably) this tree split at a weak point in its branching, as that attachment point has a narrow angle. Branches that connect with either a larger branch or a trunk which have a wider "crotch" angle are more sturdily attached and less likely to split from wind or weight stress. (The diagram on that page isn't the best, since it appears to show twin trunks that would be undesirable by itself, but the principle of how wide that crevice angle is is still applicable.)

Split wood does not mend itself back together again. Some gardeners have luck physically screwing the two halves together to keep them from re-splitting, but we generally don't recommend this because (a) it doesn't always work, (b) it creates another wound the tree cannot seal well, which risks wood decay, and (c) it creates a hazard that will damage any grinding/chipping equipment or create shards of metal when the tree dies at some point in the future and needs to be taken down (even if that's a couple decades from now).

The reason there's been little to no wilting or dieback so far is because so little of the cambium was damaged. The living layer of tissue responsible for transporting nutrients and sugars back and forth, called the cambium, lies just underneath the bark. The water-conducting tissues are just beneath that, and are naturally dead cells that act somewhat like straws drawing up water by capillary action. So, each section of this split has most of its cambium intact, which is why it hasn't dried-out and shed foliage yet, though it still might if those tissues start to die off from exposure or colonization by wood decay. Bark is one of a tree's main lines of defenses against wood decay.

Tree trunks and limbs thicken with age, so increase in weight, which means any external type of binding has to be very strong and won't likely be sufficient to keep this split from worsening or tearing further in future storms or ice/snow loads. While broken branches usually should be trimmed off, in this case that will leave a significant amount of inner trunk wood exposed to decay. Even if the tree can seal this over with wound-closure tissue produced along the wound edges, that will take several years and might not be successful. You should not coat the wound in that situation because it will hinder this process and does not help with "healing." (Plant injuries don't heal like animal injuries do,  regarding tissue mending and scarring-over, but in this sense we're talking about wound closure.)

Unappealing a solution though it may be, it might be worth it just to replace the tree since this is such a substantial break. We can't predict how long the tree will live before the split worsens. If you prefer to keep the tree for now and wait to see how it fares, you'll need to find a strong yet padded support tie that won't cut into the bark and won't constrict its expansion as the tree ages. A bungee cord might suffice if it's padded with foam or some other type of cushion, but we don't have particular advice for this approach since it's not usually done. If you do wind-up needing to replace it, look for a tree with wider crotch angles, if possible.

Miri