I have date palms, and gather the dates and sell them in the market. What is the rate of zakaah on them?

Firstly: 

The scholars are
unanimously agreed that zakaah must be paid on dates, but that no zakaah is
due unless they meet the nisaab (minimum threshold at which zakaah becomes
due), which is five wasqs. A wasq equals sixty saa’, and a saa’ is four
mudds, and a mudd is what an average man scoops up in two hands.  

See al-Mughni, 4/154 

The evidence for that in the Sunnah is the report narrated by
Muslim (979) from Abu Sa’eed al-Khudri, that the Prophet (peace and
blessings of Allaah be upon him) said: “There is no zakaah due on grains or
dates unless they reach (the amount of) five wasqs.” 

Secondly:  

The rate of zakaah that is to be paid on crops and fruits
differs according to the way in which they are irrigated. 

If they are irrigated without any expenditure, such as if
they are irrigated by rainfall or springs, or they are close to water and
their roots can drink from it, then the rate of zakaah is one-tenth. 

If they are irrigated with expenditure, such as if a machine
is needed to irrigate them and bring water to them, then the rate of zakaah
is half of one-tenth. 

This is the view of the four Imams, and Ibn Qudaamah said in
al-Mughni (4/164-166): We do not know of any differing opinion. 

What is meant by expenditure is if a machine is needed in
order to bring water to the land. This was stated in al-Mughni,
4/166. The evidence for that is the report narrated by al-Bukhaari (1438)
from ‘Abd-Allaah ibn ‘Umar (may Allaah be pleased with him), that the
Prophet (peace and blessings of Allaah be upon him) said: “For that
which is watered by the sky or a spring, or by its roots, (the rate of
zakaah) is one-tenth, and for that which is watered by a camel, (the rate of
zakaah) is half of one-tenth.” 

Al-Haafiz said: 

Al-Khattaabi said: “by its roots” means that which finds
water through its roots without being irrigated. 

“A camel” refers to a camel that is used to bring water.
Camels are mentioned by way of example here; oxen and other animals used for
the same purpose also come under this ruling. 

Shaykh Ibn Baaz said (14/74): 

Grains and fruits that
are irrigated by rain, rivers and springs, such as dates, grapes, wheat and
barley, are subject to zakaah at a rate of one-tenth. For those that are
irrigated by means of machines etc, the rate is half of one-tenth. 

It says in Fataawa al-Lajnah al-Daa’imah (9/176): 

One-tenth is due on that which is watered without any effort,
such as by rain, streams and that which drinks through its roots, and half
of one-tenth is due on that which is irrigated by artificial means such as
machines. If it is irrigated for half the year by one means and for half the
year by the other, then the rate is three-quarters of one-tenth. If it is
watered by one means more than by the other, then it should be worked out
according to the means which is used more. If the ratio is unknown, then
one-tenth must be paid.