Questioning Peat Pots

We are questioning about slow to zero growth of seedling started in peat pots. In our case it is the zero growth of castor bean (Ricinus) and Purple Angel's Trumpet (Brugmansia). Our neighbor observed slower growth on his vegetable seedlings started in peat pots compared to ones grown in plastic pots.

Mid March:

Trays of peat pots (on the right) contained castor bean seeds in the green house were set. Fortunately, since we had abundant seeds we ran out of peat pots and then used 4-inch plastic pots (on the left in white) for the remaining seeds.

Today (mid May):

The castor bean plants started in 4in plastic pots have been transplanted into 1-gallon pots and grown about 1 foot / 30 cm tall. The ones started in the peat pots? Look at the first picture. They are still under the potting mix! Two actually did come up (very late after the others were transplanted to the 1-gallon pots) but didn't last long.

Mid March:

Brugmansia seeds in one seed pod also started in peat pots (1 - 3 seeds per hole).

Today (mid May):

Brugmansias in 4-in pots, NOT from the seedlings started in the peat pots. I dug these up from the ground where I had the plants last year. NO seedling grew in the peat pots.

VEGETABLES
Mid March - Starting vegetables seedlings in peat pots. Varieties include squashes, zucchini, watermelon, melon, eggplant.

Mid May:

The seedlings made it and this is a row of squash after transplanted into the ground sometime in April. We didn't use plastic pots at all for vegetables, thus comparison is not available.

I found an information on the internet that one of the causes of slow plant growth is low temperature of soil / medium. 

"Evaporation from a medium (i.e. peat pots) tends to chill the medium quite a bit due to the evaporative cooling effect. As the peat pot warms, it draws moisture outward, the evaporation effect cools the peat (like sweating). New growers often make the mistake of adding excessive amounts of water, resulting in cold soil, poor root formation and slowed growth."

(Quoted from info source: Some Causes of Slow Plant Growth)

In our case, overwatering might be the cause since the medium looks dry almost all the time even short after watering and that worried me and drove me to watering more. 

Nevertheless, we had healthy twelve 1-gallon pots of castor bean and 2 dozens of 4-inches pots of brugmansia bought by a local nursery today! And we have more planted all around the garden, too. Sweet!