Siman 646
Presented here for study, not as a practical ruling. The Shulchan Aruch rules by Sephardi custom, the Rema's glosses by Ashkenazi custom, and the Kitzur follows Ashkenazi custom. For a practical question, ask your rabbi.
א הֲדַס שֵׁנִּקְטַם רֹאשׁוֹ, כָּשֵׁר; נָשְׁרוּ רֹב עָלָיו, אִם נִשְׁתַּיְּרוּ שְׁלֹשָׁה עָלִין בְּקֵן אֶחָד, כָּשֵׁר.
1 The Laws of the Hadas (Myrtle), 11 Seifim: If the myrtle's head is severed, it is valid. If most of its leaves fell off but three remain on one stem, it is valid.
ב הָיוּ עֲנָבָיו מְרֻבּוֹת מֵעָלָיו, אִם יְרֻקּוֹת, כָּשֵׁר; וְאִם אֲדֻמּוֹת אוֹ שְׁחֹרוֹת, פָּסוּל; וְאִם מִעֲטָן, כָּשֵׁר; וְאֵין מְמַעֲטִים אוֹתָם בְּיוֹם טוֹב, לְפִי שֶׁהוּא כִּמְתַקֵּן; עָבַר וְלִקְּטָן, אוֹ שֶׁלִּקְּטָן אֶחָד אֶחָד לַאֲכִילָה, הֲרֵי זֶה כָּשֵׁר. וּמִיּוֹם רִאשׁוֹן וָאֵילָךְ, כָּשֵׁר בְּכָל עִנְיָן (בֵּית יוֹסֵף בְּשֵׁם א"ח).
2 If its berries outnumbered its leaves and they are green, it is valid. If they are red or black, it is invalid. If their numbers were diminished, it is valid, though we do not diminish them on Yom Tov because it is considered fixing. If one sinned and picked [the berries] or one picked them off one a time to eat them, it is valid. And after the first day, they are valid in any event (Beit Yosef in the name of the Orkhot Hayyim).
ג עֲנַף עֵץ עָבוֹת הָאָמוּר בַּתּוֹרָה, הוּא הַהֲדַס שֶׁעָלָיו חוֹפִין אֶת עֵצוֹ; כְּגוֹן שְׁלֹשָׁה עָלִין אוֹ יוֹתֵר, בְּגִבְעוֹל אֶחָד; אֲבָל אִם הָיוּ שְׁנֵי הֶעָלִים בְּשָׁוֶה, זֶה כְּנֶגֶד זֶה, וְהֶעָלֶה הַשְּׁלִישִׁי לְמַעְלָה מֵהֶם, אֵין זֶה עָבוֹת אֲבָל נִקְרָא: הֲדַס שׁוֹטֶה. הַגָּה: וּפָסוּל אֲפִלּוּ בִּשְׁעַת הַדַּחַק; וְאִיכָּא מָאן דְּאָמַר בַּגְּמָרָא דְּכָשֵׁר; וְעַל כֵּן נוֹהֲגִין בְּאֵלּוּ הַמְּדִינוֹת לְכַתְּחִלָּה לָצֵאת בְּאֵלּוּ הַהֲדַסִּים הַמּוּבָאִים וְאֵין ג' עָלִין בְּגִבְעוֹל אֶחָד; וְיֵשׁ מִי שֶׁכָּתַב דַּהֲדַסִּים שֶׁלָּנוּ אֵין נִקְרָאִים הֲדַס שׁוֹטֶה, הוֹאִיל וְהֵם שְׁנַיִם עַל גַּב שְׁנַיִם וְאֵינָן כַּהֲדַס שׁוֹטֶה הַמֻּזְכָּר בַּגְּמָרָא; וְלָכֵן נָהֲגוּ לְהָקֵל כְּמוֹ שֶׁכָּתַב מַהֲרִ"י קוֹלָן וּמַהֲרִ"י איסרלן זַ"ל בִּתְשׁוּבוֹתֵיהֶם.
3 "Boughs of a dense-leaved tree" [Leviticus 23:40] talked about in the Torah are myrtle whose leaves cover its tree, with three or more leaves from one node. However, if there are two leaves at the same level, one opposite the other, and a third one above them, this is not considered dense; rather, it is considered wild myrtle. Rema: And it is invalid, even in an emergency. There is an opinion in the Gemara that it is valid. That is why in these countries, we initially use these myrtles that are imported and do not have three leaves from one node. There is one who says that our myrtles are not considered wild myrtle because they are two by two and are not like the wild myrtle in the Gemara. Therefore, our practice is to be lenient, following what was written by Mahari"i Kolon and Mahar"i Iserlin z"l in their responsa.
ד יָצְאוּ הַרְבֵּה בְּקֵן אֶחָד וְנָשְׁרוּ מֵהֶם עַד שֶׁלֹּא נִשְׁאֲרוּ אֶלָּא שְׁלֹשָׁה בְּקֵן אֶחָד, כָּשֵׁר, אֲפִלּוּ נָשִׁרוּ רֻבָּם כְּגוֹן שֶׁהָיוּ שִׁבְעָה וְנָשְׁרוּ מֵהֶם אַרְבָּעָה וְנִשְׁאֲרוּ שְׁלֹשָׁה.
4 If there were many [leaves] from one node and they fell off leaving only three in one node, it is kosher, even if the majority fell off, like if there were seven and four fell off.
ה לְמִצְוָה בָּעֵינָן כָּל שִׁעוּר אֹרֶךְ הַהֲדַס שֶׁיְּהֵא עָבוֹת, וּלְעִכּוּבָא בְּרֻבּוֹ וַאֲפִלּוּ אֵינוֹ בְּרֹאשׁוֹ (טוּר).
5 For the mitzvah, we require the whole length of the myrtle to be dense. In order to hinder fulfilling the obligation, a majority suffices (even if it is not at the top) (Tur).
ו יָבְשׁוּ עָלָיו, פָּסוּל; כָּמְשׁוּ, כָּשֵׁר.
6 If they completely dry out, they are invalid. If they are slightly dried, they are valid.
ז שִׁעוּר הַיַּבְשׁוּת, אֲפִלּוּ אִם נִפְרָךְ בְּצִפֹּרֶן, אִם עֲדַיִן יְרֻקִּים הֵם, כָּשֵׁר; וְאֵינָם נִקְרָאִים יְבֵשִׁים, אֶלָּא כְּשֶׁיַּלְבִּינוּ פְּנֵיהֶם.
7 Regarding the criterion for dryness, even if they can be disintegrated by a fingernail, as long as they are still green they are kosher. They are not considered dry, just whitened.
ח יָבְשׁוּ רֹב עָלָיו וְנִשְׁתַּיֵּר בְּרֹאשׁ כָּל בַּד מֵהַג' בַּדִין קֵן אֶחָד וּבוֹ ג' עָלִין לַחִין, כָּשֵׁר; וְיֵשׁ מְפָרְשִׁים שֶׁאֲפִלּוּ אִם מֵהַג' שֶׁבְּחַד קִנָּא יָבְשׁוּ שְׁנַיִם, וְלֹא נִשְׁאַר כִּי אִם אֶחָד לַח, כָּשֵׁר, וְהוּא שֶׁיִּהְיֶה הֶעָלֶה שֶׁהוּא מֻרְכָּב עַל שְׁנֵיהֶם.
8 If most of its leaves dried out, but there remains on each of the three branches a stem with three non-dry leaves, it is valid. Some say that if among the three on one stem two dried out and there is only one non-dry leaf, it is valid, provided that the leaf is above the other two.
ט אִם אוֹתָם עָלִים שֶׁלֹּא יָבְשׁוּ הֵם כְּמוּשִׁין, יֵשׁ פּוֹסְלִין וְיֵשׁ מַכְשִׁירִין.
9 If the leaves that did not dry out are semi-dry, some say they are invalid and some say valid.
י נִקְטַם רֹאשׁוֹ, כָּשֵׁר, אֲפִלּוּ לֹא עָלְתָה בּוֹ תְּמָרָה; וְהוּא הַדִּין לְיָבַשׁ רֹאשׁוֹ, וְיֵשׁ פּוֹסְלִין בְּנִקְטַם רֹאשׁוֹ. הַגָּה: וְטוֹב לְהַחְמִיר בְּמָקוֹם שֶׁאֶפְשָׁר בְּאַחֵר (הַמַּגִּיד), וְלֹא מִקְרֵי נִקְטַם אֶלָּא אִם נִקְטְמוּ הָעֵצִים (רַ"ן).
10 If its top is severed it is valid even if no berry grew in its place. The same is true if its top is dried. Some invalidate the severed top. Rema: It is best to be stringent in places where you can get another (the Maggid). It is not considered severed unless the wood is cut (Ra"n).
יא אִם אֵין לוֹ אֶלָּא הֲדַס שֶׁעֲנָבָיו מְרֻבִּים מֵעָלָיו, בְּיוֹם טוֹב, נוֹטְלוֹ וְאֵינוֹ מְבָרֵךְ עָלָיו.
11 If one only has myrtle whose berries outnumber its leaves, one should take it on Yom Tov and not bless on it.