TheViking said:
Mmm... nice table there, although the type is a little too small.
Then up the font size in your browser prefs

I can read it fine here point size 14 on 1600 x 1200 screen
The also lists the species into Great, Good, Difficult, Extremely Difficult at the end (Table 2). I was interested to see he had English Ivy (Hedera helix) in the V. Difficult section after reading what Gary said. Just goes to show how these things are personal and how many variables there must be.
These are his 'Great' selection:
I have
added info on availability in Britain
[NATIV]=Native to Britain and can be found growing wild
[NATUR]=Naturalised in Britain from abroad (introduced by accident or deliberately planted) and can be found growing wild
[ORNAM]= Ornamental garden introductions. Only likely to be found in gardens, parks ( especially in arboretums) or possibly as garden escapes. A few, especially the annuals, biennials may never have been introduced as garden plants.
The * notes are mine. I have not double checked this info!
Great
Achillea millefolium - Yarrow [NATIV]
Conyza canadensis - Horseweed [NATUR]
Acer negundo - Box Elder [ORNAM]
Aesculus californica - CA Buckeye [ORNAM]
Baccharis salicifolia - Mule Fat [ORNAM]
Sambucus mexicana - Blue Elderberry [ORNAM]
Sequoia sempervirens - Coast Redwood [ORNAM]
Typha latifolia - Cattail [NATIV]
Acer macrophyllum - Big-leaf Maple [ORNAM]
Artemisia douglasiana - Mugwort [NATIV]*1
Cirsium vulgare - Bull Thistle [NATIV] *2
Cytisus scoparius - Scotch Broom [NATIV]*3
Pseudotsuga menziesii - Douglas Fir [NATUR]*4
Scrophularia californica - CA Figwort
Sonchus oleraceus - Sow Thistle
*1 correctly in Britain Artemisia vulgaris
*2 Common Thistle in Britain
*3 also called Broom, Common Broom
*4 Forestry species. occasionally self seeds in Britain
You can find out more info on any of these plants at the great US Dept. of Agriculture, Natural Resources Conservation Service
Plants Database
Simon