Solarization is a method of heating moist soil by covering it with plastic sheets to trap solar radiation. In field experiments in Israel during the summer, maximum soil temperature under plastic cover at the 5-cm depth averaged 46 to 49C. No weeds emerged under the plastic cover during solarization and weed emergence was reduced after its removal. The heating effect from solarization decreased with soil depth. Concentration of O2 in soil under plastic was similar to that in uncovered controls, but the concentration of CO2 was markedly higher than in control soil, rising up to 2.4%. Higher temperatures and better residual weed control were produced by transparent than by black plastic, with best results from thin (0.03 mm), transparent polyethylene. Under Israeli summer conditions, 2 to 4 weeks of solarization produced effective control of annual weeds that was still appreciable after 1 yr. Narrow sheets of 20 to 50 cm produced effective weed control in bands. on soil irrigated once before placing the plastic sheets, there was no need to irrigate during solarization. The response of weed species to solarization differed. Many annual weeds, both summer species such as pigweed (Amaranthus spp.) and common purslane (Portulaca oleracea L.) and winter species as henbit (Lamium amplexicaule L.) were well controlled by solarization. Broomrape (Orobanche crenata Forsk.) was controlled in one experiment. on the other hand, horseweed [Conyza canadensis (L.) Cronq.] and bull mallow (Malva niceaensis All.) were relatively resistant, and established perennials escaped the treatment.

Summary. Single tubers of Cyperus rotundus L. were planted at intervals over the year. Plant growth was slow and sprouting of tubers was inhibited at temperatures below 20°C, but tubers overwintered at temperatures above freezing point. In the warm season, plant growth and tuber formation rate closely followed air temperature and tubers were forming within 1 month from planting. No inflorescence appeared during the cool season. In autumn‐planted C. rotundus grown in containers, the ratio of aerial to subterranean weight decreased from 1·1 in December to 0·2–0·4 in summer. The weight of tubers in mid‐summer was about 10 times more than that present in December. Tubers formed at ail times of year and at various locations on plants sprouted readily in laboratory tests (76–100% sprouting). C. rotundus planted in March at wide spacings was grown in field conditions free of other plant competition for 20 months. Within 2 months the plants had spread to 90 cm. At the end of the first and the second summer of growth, the mean area of one plant was 7·6 m2 and 56·7 m2, respectively, and patches had expanded then by 2·8 m and 5·4 m, respectively, from the initial shoot. After 20 months of growth all tubers were present within the 0–40 cm soil depth, 60–70% of them in the 0–20 cm layer. About 30% of the tubers were within 1 m and 60% within 2 m of the plant centre. Under the patch centre there were about 1000 tubers per m2 with 0·3 kg dry weight; in the upper 20 cm more than 3500 tubers weighing 0·9 kg were present per m3 of soil. Croissance, formation de tubercules et propagation de Cyperus rotundus L. issu de tubercules uniques

Summary. Small plants of Sorghum halepense (L.) Pers. were planted in the field and grown without competition from weeds and crops for 2½ years. The plant spread by means of subterranean rhizomes from which aerial shoots developed at increasing distances from the plant centre, and by tillering around these aerial shoots to form clumps. No preferential direction of expansion was detected and established patches developed an approximately circular shape. Aerial growth stopped completely in the cold season. During the warm season the mean area increment amounted to 1·3 m2/month and was similar in both years of observations. After 2½ years of growth, patches had extended up to 3·4 m from the initial sprig and had a mean area of 17 m2. In the second summer of growth, shoot density reached 190/m2 and mean seed production was 84 g, or 28 000 seeds per plant. About 80% of rhizome dry weight was present in the upper 20 cm of soil and rhizomes did not penetrate deeper than 40 cm. About 80% of rhizome dry weight was present within 1 m radius from the plant centre. Croissance dans l'espace de Sorghum halepense (L.) Pers.

Tillering, flowering, and rhizome formation of johnsongrass(Sorghum halepense(L.) Pers.) started approximately 2 months after planting seeds or single-node rhizome fragments in the warm season. The minimum temperature for rhizome formation was between 15 and 20 C. Inflorescences were produced only from May to November. No causal relationship was found between flowering and rhizome formation. The bud activity of new rhizomes, as assessed by sprouting of single-node fragments, ranged between 48 and 100%. Sprouting was nil at 10 C, slow below 20 C, and maximum at 28 C. Seed germination required a temperature about 10 C higher than rhizome sprouting. While the initial leaf production of seedlings was more rapid than on plants produced from rhizomes, other differences in growth and development between seedlings and rhizome plants were of only limited practical significance.

Dried subterranean organs of Cynodon dactylon, Cyperus rotundus and Sorghum halepense were incubated in light and heavy soils for 1, 2 or 3 months. After removal of the decayed plant material, the residual bioactivity ofthe soil was assayed by barley sown directly into the soil and by a barley radicle-length test on an ethanolic extract of the soil. The two assays gave similar results. In general, the inhibition of growth was proportional to the concentration of plant material in the soil, and was greater in light than in heavy soil. Greater inhibition was caused by C. rotundus and S. halepense than by C. dactylon. A few instances of growth stimulation were recorded. Activity biologique de residus souterrains de Cynodon dactylon L., de Sorghum halepense L. et de Cyperus rotundus Z... R^sum^. Des organes souterrains d&s^ch6s de Cynodon dactylon, Cyperus rotundus et Sorghum halepense ont €l6 enfouis dans des sols 16gers et dans des sols lourds pendant 1, 2 ou 3 mois. Apr6s avoir enlev6 le materiel v6g6tal d^compos^, l'activit^ biologique r&iduelle du sol a 6t6 6valu6e avec de I'orge sem^e directement dans le sol et par un test portant sur la longueur de la radicule de I'orge dans un extrait ^thanolique dusol. Eng^n^ral, l'inhibition de la croissance fut proportionnelle i la concentration du materiel vdg^tal dans le sol, et fut plus grande en sol 16ger qu'en sol lourd. L'inhibition provoquee fut plus importante avec C. rotundus et S. halepense qu'avec C. dactylon. Quelques cas de stimulation de croissance furent enregistrfe. Die biologische Aktivitat unterirdischer Teile von Cynodon dactylon L., Sorghum halepense L. und Cyperus rotundus L. Zusammenfassung. Getrocknete unterirdische Organe von Cynodon dactylon, Cyperus rotundus und Sorghum halepense wurden in leichten und schweren Boden uber ein, zwei und drei Monate inkubiert. Danach wurde das verrottete Pflanzenmaterial entfernt und die verbliebene biologische Aktivitat des Bodens mit direkt in den Boden gesater Gerste und mit Hilfe von athanolischem Bodenextrakt und einem Gerstenwurzeltest gemessen. Im allgemeinen war die Wuchshemmung proportional der Konzentration des pflanzlichen Materials im Boden und war starker im leichten als im schweren Boden. C rotundus und S. halepense verursachte starkere Wuchshemmung als C. dactylon. Es wird liber einige Falle berichtet in denen das Wachstum stimuliert wurde.