There use to be a tree shading this Hosta.
Will it survive till the Fall or should I move it.? I did want to split it as well.
Thank-you for contacting Toronto Master Gardeners with your question.
Your poor hosta certainly seems to be suffering from sun scorch. The ones with large, heavily corrugated, yellowish leaves such as yours are usually pretty good in a sunnier situation, but this one is getting too much sun! As you note, early fall or spring are better times to transplant and divide plants than mid-summer. Cool temperatures are preferable so that the transplanted root systems have plenty of moisture while they recuperate. The good news is that hostas are very tough perennials and your is unlikely to be killed off whether you decide to leave it in place until September or dig it up and transplant it now. The key thing in either scenario will be to keep it very well watered through the summer months.
In making your decision, you need to consider a couple of factors. If you wait until fall, your plant may have been weakened by its full sun summer ordeal. While it will probably survive, it may take a while to recover, especially since it also has to go through the stress of transplanting. This resource gives some more tips on growing hostas in the sun:
https://www.thespruce.com/which-hostas-can-grow-in-sun-1402799
There are two issues with going ahead with the transplanting now. The first is the upcoming stress of the summer weather for the new transplants. The second is that the leaves of your large hosta are pretty fully grown. Many leaves will inevitably be damaged by the digging up, dividing and re-planting exercise. On the whole, I would favour digging up your hosta now and dividing it into just a few large pieces. At least you can ensure the plants have protection from the full afternoon sun! The resources below discuss the best approaches to dividing hostas. While sources do suggest digging up your plant and then teasing the roots apart, I find that hostas have such a sturdy, dense root clump that using a large, sharp knife to cleanly cut sections is much more effective.
https://www.gardeningknowhow.com/ornamental/foliage/hosta/dividing-hosta-plants.htm
https://www.nhhostas.com/dividing-hostas/
The Toronto Master Gardeners Gardening Guide below gives good general information on dividing perennials such as Hosta:
Regardless of the timing you choose, please make sure you give your hosta (or hostas) enough water to keep them strong!