264 Encyclopedia of Gardening 



The Aspen thrives on clay. The white Poplar likes a moist soil. 

 Propagation is by seeds, la^-ers, and grafting, but is generally left 

 to nurserymen. 



Poppy. See Papaver. 



Poppy, Californian, Platystemon californicus. 



Poppy, Horned, Glaucium luteum. 



Portugal Laurel. See Laurel and Prunus. 



Portulaca (portulac-a, from porto, to carry, and lac, milk. Ord. 

 Portulaceae) . Charming half-hardy annuals, with single and double 

 flowers of brilliant and varied colours, thriving in sunny spots. For 

 culture, see Annuals Half-hardy. P. oleracea is the Purslane, a 

 kitchen herb. 



Potato (Solanum tuberosum. Ord. Solanaceae). See Kitchen 

 Garden. 



Potato Onion. See Kitchen Garden. 



Potentilla, Cinquefoil (potentil-la, from potens, powerful, in allu- 

 sion to the supposed medicinal properties. Ord. Rosaceae) . Charm- 

 ing plants, several good species and hybrids being hardy and suitable 

 for the border and rock garden. They have Strawberry-like foliage 

 and brilliant single or double flowers. They will thrive in any 

 good garden soil; for the alpines it should be gritty. Propagation 

 is by seeds or division in spring. Ambigua, yellow, 6 ins. high, 

 early summer (Botanical Magazine, t. 4613) ; fruticosa, a shrub 3 to 

 4 ft. high, yellow flowers in summer; nepalensis (syn. formosa), 18 

 ins., pink flowers in summer; and nitida, 4 to 6 ins., rose flowers in 

 summer, are a few of the best species. 



Pots. See Flower-pots. 



Potting. A gardening operation well worthy of study. As a 

 rule, the first pot of a young plant, whether from seed or cutting, 

 should not exceed 3 ins. across (for this and other sizes, see Flower- 

 pots). Unless in special circumstances the shift from this may be 

 to a 5 -in., from that to a 7-in., and from that to a Q-in. or lo-in. if 

 further repotting is necessary. Economy in time and material may 

 be effected by shifting from a 5 -in. to an 8 -in. if watering is carefully 

 done until the plants are rooting freely again (see Watering), other- 

 wise the soil might become sour. It is desirable that the pots should 

 be clean inside as well as out ; if dirty the roots bind on the bottom 

 and are torn in repotting. Generally speaking, plants need re- 

 potting when roots show freely at the drainage hole. The fresh 

 soil should be pressed firmly round the roots. See also Drainage and 

 Soil. 



Pricking-out. A term applied to the operation of transplanting 

 seedlings from the pans or boxes in which the seeds were sown. It 

 should be done before the plants spoil each other by crowding. 



Prickly Pear (Opuntia). See Cactus. 



Primula (Primrose, Polyanthus, Auricula. Ord. Primulaceae). 

 A large and very important genus, including, as it does, one of our 



