3 letters:
ede

4 letters:
bead, bede, cede, deed, feed, he'd, heed, lead, mead, need, read, reed, reid, seed, teed, we'd, weed

5 letters:
aidid, alkyd, bleed, brede, breed, canid, creed, freed, fried, glede, gleed, greed, keyed, knead, kneed, meade, plead, q.e.d, she'd, skied, snead, speed, steed, swede, teiid, treed, tweed

6 letters:
accede, agreed, axseed, exceed, impede, indeed, lipide, pitied, rashid, recede, reread, reseed, screed, secede

7 letters:
aniseed, arctiid, benweed, concede, cudweed, decreed, dyeweed, fanweed, glochid, gumweed, hayseed, hogweed, implead, inbreed, limeade, linseed, lygaeid, mayweed, misdeed, mislead, misread, nitweed, oilseed, pigweed, precede, proceed, ragweed, seaweed, succeed, tarweed

8 letters:
agueweed, airspeed, antecede, aristide, bindweed, blueweed, bur reed, capeweed, copyread, dilaudid, duckweed, fireweed, flaxseed, ganymede, gulfweed, hawkweed, ironweed, knapweed, knotweed, lockheed, locoweed, markweed, milkweed, moonseed, overfeed, pay heed, pineweed, pokeweed, pondweed, rapeseed, reduviid, richweed, rockweed, silkweed, soapweed, stampede, tickseed, tickweed, wartweed

9 letters:
benniseed, blackseed, brookweed, broomweed, bugleweed, candyweed, cane reed, catchweed, centipede, cerripede, chafeweed, chaffweed, cheerlead, chickweed, clearweed, crazyweed, dill seed, dill weed, disagreed, fern seed, free-reed, frostweed, gelechiid, greenweed, gypsyweed, horseweed, intercede, jewelweed, line feed, matchweed, millepede, millipede, nosebleed, orangeade, pearlweed, pedigreed, poeciliid, proofread, retrocede, rosinweed, sand reed, saturniid, siegfried, skunkweed, smartweed, snakeweed, spoonfeed, stinkweed, supersede, symphilid, take heed, underfeed, waterweed

10 letters:
beetleweed, beggarweed, bitterweed, black bead, bottlefeed, butterweed, cancerweed, cankerweed, carpetweed, clammyweed, cottonseed, cottonweed, crazy weed, creep feed, crossbreed, ditch reed, force-feed, force feed, ghost weed, giant reed, gravelweed, guaranteed, half-breed, hemerobiid, high-speed, interbreed, interplead, jimsonweed, lymantriid, poppy seed, quirk bead, rabbitweed, scarabaeid, silverweed, sneezeweed, spoon-feed, title deed, trust deed, tumbleweed, velocipede, velvetweed, weak-kneed, white bead

11 letters:
bottle-feed, camphorweed, canary seed, cancer weed, celery seed, chickenfeed, common reed, double reed, edible seed, fennel seed, french weed, gracilariid, groundspeed, knock-kneed, light speed, megatheriid, pumpkinseed, rattle weed, sesame seed, tettigoniid, thimbleweed, vinegarweed, walter reed

12 letters:
caraway seed, chicken feed, devil's weed, english-weed, harris tweed, jumping seed, mustard seed, pickerelweed, polecat weed, prickle-weed, pumpkin seed, scorpionweed, star aniseed, trombiculiid, trumpet weed, unguaranteed

13 letters:
at full speed, false ragweed, great ragweed, mortgage deed, pickerel weed, poke milkweed, pressure feed, scorpion weed, silkworm seed, star-duckweed, stinking weed

14 letters:
alligator weed, balm of gilead, butterfly weed, common ragweed, coriander seed, fenugreek seed, great duckweed, jamestown weed, marsh milkweed, pineapple weed, safflower seed, showy milkweed, sunflower seed, swamp milkweed, water flaxseed, white milkweed

15 letters:
common duckweed, execution speed, house centipede, lesser duckweed, mead's milkweed, orange milkweed, poison milkweed, rheumatism weed, turpentine weed, western ragweed

16 letters:
consumption weed, corn gluten feed, garden centipede, rattlesnake weed

17 letters:
perennial ragweed

18 letters:
horsetail milkweed

19 letters:
egyptian paper reed