Tight sprouts are firm, round, and about 1 to 1½ inches across, with leaves wrapped closely like a miniature cabbage. Loose sprouts look like open tufts of small leaves, feel soft when squeezed, and often taste bitter. Check where on the stalk the problem is. Sprouts form from the bottom up, so lower sprouts that developed in late-summer heat can be loose while the upper ones, forming as nights cool, tighten up. Peel a few open while you are looking: aid colonies can build up inside sprouts, loose or tight, where you can't see them from outside.
Heat is the main cause. Illinois Extension notes that sprouts developing in hot weather, whether after spring seeding or during a warm fall, often fail to form compact heads, and that sprouts maturing in heat or dry soil are more likely to taste bitter. Brussels sprouts grow best in cool or even lightly frosty weather. A crop early in spring often reaches the sprout stage in midsummer, which is the wrong weather for tight sprouts. In zones 8 and warmer, even a fall crop may not get a long enough cool spell.
Fertility and care come next. Too much nitrogen late in the season tends to push leafy growth instead of firm sprouts, so stop feeding once sprouts start to form. Dry soil while sprouts are sizing up adds stress. Tall plants that lean or fall over in wind or loose soil can stall. Harvest timing matters as well: sprouts left on the stalk well past full size can open and yellow, so pick the lower sprouts as they reach 1 to 1½ inches rather than waiting to harvest the whole stalk at once.
If your first fall frost is still several weeks away, leave the plants standing. Sprouts that form as nights cool are likely to be tighter and sweeter, and the plants tolerate several frosts. About 3 weeks before you plan to harvest, cut off the top growing point; Illinois Extension notes this stops new leaf growth and directs the plant's energy into the sprouts already forming. Keep watering through dry spells. Loose sprouts that have already formed are still edible; shred them for a slaw or cook them as you would other greens.
Next season, plan backward from fall. Illinois Extension suggests setting out transplants in early to midsummer, at least 90 to 100 days before your first fall frost, so most sprouts fill out in cool weather. Choose a variety whose fits your season; faster help in short-season areas. Firm the soil at planting, hill soil or stake tall plants, nitrogen once when plants are about 12 inches tall rather than repeatedly, and keep the soil evenly moist through summer. In the warmest zones, aim for sprouts that mature in the coldest part of winter.
- AphidSoft, clustered insects on new growth causing curled leaves and sticky honeydew.
- Imported CabbagewormRagged holes in brassica leaves with pale green caterpillars and green frass nearby.
- Fruit CrackingTomatoes crack at the stem end or shoulder, cabbage heads split, and carrots split lengthwise — usually after heavy rain follows dry weather.
- White-tailed DeerRagged, torn foliage and missing plants from the top down — hoof prints nearby confirm the cause.
- Root-Knot NematodeStunted, wilting plants with characteristic knobby galls on the roots. Worst in sandy soil and warm climates.
- Can you give a plant too much nitrogen, and what does it look like?Yes — excess nitrogen produces very dark green, lush leafy growth at the expense of flowers and fruit, and in high concentrations can burn roots and cause wilting.
- How can I extend my growing season in fall?Row cover fabric, cold frames, and switching to cold-hardy crops are the three most reliable tools for extending production 4–6 weeks past your first fall frost.
- Why did my broccoli skip making a head and go straight to flowers?Broccoli bolts when it experiences heat or stress during head formation — once it starts flowering, the head is unsalvageable, but timing adjustments can prevent it next season.