One of the 39 prohibited Melachot (activities) on Shabbat is "Kosher"-tying knots. Maran discusses these Halachot in Siman 317. There are three categories of knots. Some knots are prohibited from the Torah, some are prohibited M’drabanan (Rabbinically), while others are permitted altogether. In order to be prohibited from the Torah, a knot must have two qualities. First it must be "Ma’aseh Uman"- a professional knot requiring special skill to tie. Second, it must be "Kesher Shel Kayama"- a permanent knot. While there are different opinions as to how long the knot has to last, the opinion of Rambam is a minimum of seven days. A knot which has only one of these qualities-either "Uman" or "Kayama"- is prohibited M’drabanan. If the knot is neither, i.e. it is not a professional knot and is not designed to last more than seven days, it is permitted to tie on Shabbat.
Maran also rules that a simple but permanent knot, which is generally prohibited M’drabanan, is permitted for the sake for a Misva. One application of this leniency is when Ashkenazim tie the Sefer Torah with a scarf. They use a simple knot that may last for more than a week, e.g. when tying the Sefer of Rosh Hodesh.
The question was asked whether this leniency applies to tying the Lulav on Succot, if one forgot to do so before Yom Tob. Generally one ties the lulav with a simple knot, which may indeed last more than a week. So it would seem that it should be permitted to tie it for the sake of Misva. The only problem with this logic is that tying the Lulav is not considered a Misva. The Halacha is not in accordance with Rabbi Yehuda in the Mishna who holds that it is a bona fide Misva to tie the Lulav; it is only an enhancement.
Another approach to permitting tying a Lulav could be based on the Rambam who holds that any items edible to humans or animals are exempt from the prohibition of tying. Since the leaves of the Lulav are edible to animals, it would be permitted to tie it on Yom Tob. The only problem with this is that Maran does not cite this leniency of the Rambam. Moreover, the Rif and the Rosh argue with the Rambam on this point. Since Maran generally rules in accordance with the majority ruling of the Rif, Rambam and Rosh , it seems that he disagrees with the Rambam’s leniency.
Thus, Hacham Ovadia (Hazon Ovadia Vol. 5) and Menuhat Ahaba (Vol. 3) conclude that the prohibition of tying applies also to edible items. Therefore, one may tie his Lulav on Yom Tob with a simple knot only if he intends to untie it during Hol Hamoed. That way, he has both lenient factors in his favor; it’s simple and non-permanent. It should be mentioned that Yalkut Yosef (Moadim) states that one may rely on the Rambam’s opinion. Nevertheless, Hacham Ovadia, clearly disagreed.
Based on this, one should also be careful not to tie knots in string licorice, and the like, and then store them.
SUMMARY
One may tie a simple, non-permanent knot on Shabbat.
It is permitted to tie a simple permanent knot for the sake of a Misva.
The prohibition of tying knots applies to food, as well.
A Lulav may be tied on Yom Tob with a simple knot, only if the intent is to untie it on Hol Hamoed.
