Our sages learnt from the verse "And ye shall take you on the first day... and boughs of thick trees" ( Vayikra 23, 40) that one of the four species is Hadas - myrtle . We take 3 branches of Hadas that have 3 leaves growing from each point.
• Hadas grapes / fruit - If the Hadas has more fruit than the leaves, it’s Pasul. Purple, black or red fruit aren’t the color of the Hadas and are Pasul. (The Mishna Berurah doubted about green fruit). The fruit can be taken off (not on Yom Tov) and the Hadas will be Kosher. .
• The myrtle leaves should cover the branch (It is preferable that the bottom leaf will reach the leaf above, since the Mor Uktziah said that this is necessary, though the Chazon Ish and Bikkurei Yaakov argued ). Sometimes, when the myrtles are in the shade, the leaves open, but it is still Kosher.
Hadas Shoteh is a Hadas without triple leaves and they don’t cover its branch. Such a Hadas is Pasul.
• The Hadas should be "Avot" - thick which means that all three leaves should start from the same height. The Chazon Ish holds it is enough if you can draw a line through the leaves stems.
Some hold that all three leaves should start from exactly the same line (Rashi), such a Hadas is considered better. Rabbi Shlomo Zalman holds its enough that part of the stem meets, so they can be in a line of about 2 mm thick. It is better that all the Hadas is triple and "Avot" but also mostly Avot is Kosher.
• Length - the branches of Hadas should be 3 Tefachim - 24 cm according to the Grach Naeh and according to the Chazon Ish - 29 cm. The length does not include the green leaves at the end.
• The size of the leaves - the leaves should be at least as big as a fingernail or as up to the joint. They shouldn’t be too small.
• Truncated - we should try to get Hadasim that aren’t truncated , though some hold its Kosher.
• Height in relation to the lulav - the spine of the lulav should be a Tefach (8-10 cm) longer than the Hadas and willow. Therefore, we should not buy a short Lulav and ‘giant’ Hadasim.
• Keeping the Hadas - it is better to keep the Hadasim in a sealed bag in the refrigerator and not in a wet towel since that causes the leaves to fall.