Growing arugula in zone 4b

Last spring frost in zone 4b typically falls between mid-May and late May, depending on elevation and local cold air drainage. Arugula tolerates light frost and can be direct-sown outdoors four to six weeks before that date, usually early to mid-April, with row cover providing insurance against hard freezes. Germination begins reliably at soil temperatures above 40°F. Days to first harvest run 35 to 45 days depending on variety and temperature. A second succession can go in two to three weeks after the first. As summer heat builds in July, bolt risk rises sharply. Fall plantings timed for mid- to late August will mature before the first hard frost in early September, giving a second quality harvest window when temperatures drop back into the optimal range.

The main adaptation in zone 4b is protecting early spring plantings from late frost events, which can occur into late May. Floating row cover rated for 4 to 6 degrees of frost protection is sufficient and also speeds soil warming by a week or more. Remove or vent cover during warm midday spells once temperatures climb, since excess heat under cover accelerates bolting faster than ambient conditions would. Downy mildew pressure increases in cool, wet springs with limited air circulation; spacing plants at least six inches apart and avoiding overhead irrigation in the evening reduces leaf wetness and limits infection. Wild Rocket (Sylvetta) is slower to bolt than Astro and holds better leaf quality into warmer stretches, making it the more reliable choice for growers who want to extend the spring window. Astro matures faster and suits succession plantings better when the goal is high leaf volume early in the season.