The hardiest seeds (lettuce, onions and parsnips) can be planted out now that soil temperatures have reached 35 degrees F on the Front Range. Soil temperatures at the online Ft. Collins soil temperature website have been bouncing around between 36 and 39 degrees F since the beginning of the month. Soil temperatures warm on sunny, 50 to 60 degree days and cool in cloudy/snowy weather.
Parsnip seed is notoriously slow to germinate. It may work well to plant them with radishes to gain double use from the area. Quick maturing radishes can be harvested in 20 to 30 days and parsnips require the whole season (100 to 120 days depending on variety). 'Hollow Crown' is an heirloom variety (heirlooms are open pollinated varieties that have been in cultivation 50 years or longer). Other parsnip varieties are 'Lancer', 'Albion' and 'Javelin'.
Planting now means soil should have been fall prepped. Last season's dead plants should have been carted away or composted and the soil tilled and left rough to further break up during winter freeze-thaw. Any compost should have been added then. Note if you grew a winter cover crop, it will require 30 days after tilling under before you can plant vegetables.
Other spring crops require a 40 degree F soil temperature for seed to germinate. These include spinach, kale, beets, carrots, peas and more. Hold off on planting them for now. Of course if you have a soil thermometer and have reached forty at your Front Range Colorado location, proceed with planting.
Photo credit: Parsnip and Lettuce seed packets - Carl Wilson