It is pretty much accepted today that the tares Jesus spoke of are darnel of the genus Lolium. Cephalaria syriaca is also a possibility mentioned by some researchers, but based on archeological studies, its evident lack in Biblical times would seem to indicate it is not the tare of the Bible. Secular dictionaries also define the tare as most likely being darnel:

"Darnel is a weed grass (probably bearded darnel or Lolium temulentum) that looks very much like wheat until it is mature, when the seeds reveal a great difference. Darnel seeds aren't good for much except as chicken feed or to burn to prevent the spread of this weed" (World English Dictionary).

Easton's Bible Dictionary describes "tares" as follows:

"the bearded darnel, mentioned only in Matt. 13:25-30. It is the Lolium temulentum, a species of rye-grass, the seeds of which are a strong soporific poison. It bears the closest resemblance to wheat till the ear appears, and only then the difference is discovered. It grows plentifully in Syria and Palestine."

The American Heritage® Dictionary of the English Language: Fourth Edition 2000 defines "soporific: as:

ADJECTIVE: 1. Inducing or tending to induce sleep. 2. Drowsy.
NOUN: A drug or other substance that induces sleep; a hypnotic.

Smith's Bible Dictionary offers these comments on the "tares":

"There can be little doubt that the zizania of the parable, #Mt 13:25 denotes the weed called "darnel" (Lolium temulentum). . . . The grains of the L. temulentum, if eaten, produce convulsions, and even death."

A very interesting note I found on Botanical.com concerning darnel was this:

"The admixture of the grain with those of the nutritious cereals amongst which it is often found growing should be guarded against, as its properties are generally regarded as deleterious. Gerard tells us: 'the new bread wherein Darnel is eaten hot causeth drunkenness.' When Darnel has been given medicinally in a harmful quantity, it is recorded to have produced all the symptoms of drunkenness: a general trembling, followed by inability to walk, hindered speech and vomiting. For this reason the French call Darnel: 'Ivraie,' from Ivre (drunkenness); the word Darnel is itself of French origin and testifies to its intoxicating qualities, being derived from an old French word Darne, signifying stupefied. The ancients supposed it to cause blindness, hence with the Romans, lolio victitare, to live on Darnel, was a phrase applied to a dim-sighted person.

"The alleged poisonous properties of Darnel are now generally believed to be due to a fungus."

Here's the Scripture from the Parable of the Wheat and the Tares:

Jesus said,

Mat 13:24  Another parable put he forth unto them, saying, The kingdom of heaven is likened unto a man which sowed good seed in his field:
25  But while men slept, his enemy came and sowed tares among the wheat, and went his way.
26  But when the blade was sprung up, and brought forth fruit, then appeared the tares also.
27  So the servants of the householder came and said unto him, Sir, didst not thou sow good seed in thy field? from whence then hath it tares?
28  He said unto them, An enemy hath done this. The servants said unto him, Wilt thou then that we go and gather them up?
29  But he said, Nay; lest while ye gather up the tares, ye root up also the wheat with them.
30  Let both grow together until the harvest: and in the time of harvest I will say to the reapers, Gather ye together first the tares, and bind them in bundles to burn them: but gather the wheat into my barn.

Source: deceptioninthechurch.com